I doubt there is any gardener who has ever bought a new greenhouse who did not wish they had got a bigger one . I should know; I started with a small lean-to greenhouse, intended mostly for propagation, of course I managed to squeeze in a few tomato and pepper plants. Well this soon showed itself to be so useful I got a second-hand eight by twelve foot greenhouse.
(A word to the unwise; although it may be considerably cheaper to buy second-hand there are other costs involved. Many replacement panes, clips, fastenings, seals and the base all had to be found, not easy as I had bought an obsolete model. Furthermore the amount of time to dismantle, move, clean and reassemble the parts was astronomical. I'm sure I'd have saved a lot of effort and time if I'd bought a new one. Especially as this might have come with instructions, and although I had taken the old one to bits all the indelible markings I had made had washed off!)
Becoming greedier still in my search for more space I put up my first polythene covered walk in tunnel. Although this was considerably larger at fourteen foot wide and twenty foot long it cost less than the second-hand greenhouse and gave far more usable space per pound. However I immediately noticed that although some crops preferred being under plastic others were not so happy and did substantially better under glass. It was hard to make fair comparisons though, the soils and aspect were different and both were crowded. It was fairly obvious however that in both places I did not give the plants enough space.
Within a few years as my edible plant collection grew I had to move to a larger, twenty by forty foot polytunnel. This seemed so cavernous I never dreamed I'd be able to fill it, yet within another few years I had to move onto an even bigger twenty four by sixty foot model! Admittedly this is beyond the scope of many gardeners just because of it's size but the cost was amazingly low. Brand new the lot was just over a thousand pounds. And putting it up was not as difficult as I thought except for putting on the plastic cover which took a dozen friends to stop it parasailing into the next county.
Over the two decades I have been growing under cover I have firmly come down on the side of the polytunnel because of it's sheer size and economy. No more struggling past crowded plants knocking them off the staging as you go. No more bending double so you don't catch your head on the glass. Indeed in a big tunnel there is so much room you can move in the armchair and kettle. Whereas a greenhouse is an adjunct to the garden and a lovely place to be it is almost always cramped. It is hard to linger in a greenhouse simply because they're so small. A tunnel is a garden!. With a tunnel there is so much space available for so little money you can really indulge yourself whatever the weather outdoors.
However few have the luxury of enough space to indulge in as large polytunnels as I've gone to, though where space is available economics dictates their use as glass of any size becomes exorbitantly expensive. At the other end of the scale in the smaller backyard garden a glass greenhouse is usually gone for as their scale and size is more suitable and a small polytunnel has no great advantage over glass. Yet really the choice should be made depending on what you want to grow as in fact the two forms of cover are not the same in other ways than just the obvious one of area bought per pound spent.
The first and most obvious difference is that under plastic there is more light which is more diffused. There is very little solid structure but the plastic itself is not as clear as glass. (It also stops different wavelengths, I believe you are more in danger from sunburn under plastic than glass.) This diffuseness is much appreciated by the melons, cucumbers and salad crops all of whom thrive under plastic where I have never needed to add shading whereas under glass the cucurbits easily get scorched and suffer more from red spider mite. However this self same diffusion works against other crops; tomatoes do not really like it very much, I reckon they much prefer the brighter conditions under glass, likewise aubergines and peppers, and citrus bushes, also seem to prefer glass to plastic. But it is hard to tell as these crops all respond to humidity as well as to light intensity.
Under plastic there is usually going to be much more humid conditions than under glass. Unless extra (plastic) insulation is used in a glass greenhouse then there are vast numbers of tiny chinks and draughts. This makes the air change more often in a greenhouse and they dry out more than a polytunnel of the same size with the same contents. (However after some years of insufficient watering it is possible for a polytunnel to become extremely arid, if such is desired.) High humidity is appreciated by some crops and it does help keep red spider mite down but unfortunately moulds and rots are favoured by it, peppers fare particularly badly. Also in the very humid conditions under plastic I've seen oedema on aubergines, Physalis, peppers and even on potato leaves. This is a blistering, it looks ghastly but is merely a reaction to too much humidity. I have rarely seen this occurring under glass.
The high humidity under plastic is caused by the relatively low rate of exchange of the air. It is possible to make a polytunnel much more airtight than a greenhouse which is great for retaining heat but causes a problem. Air contains only a very limited amount of carbon dioxide, if the tunnel, or greenhouse, is tightly shut up then no new air can enter and all the carbon dioxide used up. Commercial growers supplement this gas to increase yields. I've found it makes little difference under glass but a great deal under plastic where a few demijohns of fermenting beer provide the gas with little effort.
In full summer when over-heating is a problem then the greenhouse has a distinct advantage with relatively cheap and simply fitted automatic vents. A tunnel could be so equipped but is more expensive and most rely on the gardener to open and close the ends. Automatic electric fans in the ends are an option and some tunnels have side vents which can be exposed improving air flow no end, but rarely automatically.
Plastic gets heavy condensation on the inside of the skin when it is colder outside. Condensation occurs on glass, but not to quite the same extent. On glass the drops run down and may drip on the plants from the junctures of panes. On plastic the drops fall from all the flat centre and only run down the curved sides, except when it is windy when the flapping shakes them off all over. In general then there is more dripping under plastic, which most plants detest, especially in winter. Thus it is harder to grow, and especially to over-winter, plants under plastic that must be kept bone dry.
Heating is a moot point, I cannot say which is more expensive as it depends on so many factors. The air is kept inside plastic more easily but the plastic itself lets more heat escape than glass which is a better insulator. On the other hand although glass is a better insulator the bulky aluminium greenhouse frame conducts much heat away whereas the frame in a polytunnel is proportionately small and it's heat losses almost irrelevant. With a greenhouse you are heating a small space and with a tunnel a large one which has the biggest effect of all. In the end both cost an arm and a leg to keep warm, and the bigger of either costs the more.
As heating is very expensive I found a good compromise was to double glaze my polytunnel with another inside. (See Kitchen Garden January 2000) This cuts down the light significantly but saves heat. I add artificial light in winter to replace that lost to the extra layer of plastic. Unfortunately as the light is added to strengthen the midday sun and to extend the daylight it is using full price electricity whereas the heating is mostly required at night which uses cheap rate thus the savings are not very great. However I am trying to grow many tropical crops which sulk in the dark.
Certainly it is easier to insulate a greenhouse as the fittings are available. It is also easier to fit shelves, staging or almost anything else in a greenhouse as the framework can be utilised. The lack of easily fitted staging in a polytunnel is a problem, getting the plants up higher is a great advantage for both heat and light and is simpler in most greenhouses. In a polytunnel staging must be free standing although wires can be strung and used for tying up plants.
Probably the greatest advantage of glass is that it is permanent and with a wash occasionally is kept crystal clear. Plastic slowly becomes less transparent, is harder to keep clean and after four years or so becomes brittle, splits, tears and flaps off. Repairs prove very temporary so the whole skin must be replaced which is costly and a hassle. (The plastic lasts longer if tightly fitted, eventually going first where it touches supports, using the special insulation sold as tape adds a year or so of life.) At this point it is sensible to move the polytunnel frame to a new site giving the crops new soil. Of course this depends on there being sufficient land available, in a small garden it may not be possible. But you can't move a greenhouse as easily as a polytunnel.
Indeed if land is available then there is a good argument for putting up another new polytunnel elsewhere when the plastic cover on the first goes and converting the old frame and site into a fruitcage. I covered the top of mine with plastic netting and used galvanised chicken netting for the ground level run where most mechanical damage occurs. The internal space is voluminous, the cost, even if specially bought, much better than any commercial cage and the height in the middle is terrific.)
Aesthetics must eventually come into the equation; few of us admire polytunnels as garden ornaments as they are starkly utilitarian. An attractively made greenhouse can sit more sympathetically in most gardens, and of course can even be made part of the house and add to it's value -which I doubt a polytunnel ever could. And this is no small advantage; walking straight from your living room into your greenhouse/conservatory does make for more attention and better results. Add to this the ease with which a greenhouse attached to the home can be heated direct from the house's central heating and it becomes hard to beat.
To summarise; in a small garden it is sensible to choose a greenhouse, especially for tomatoes. But if more space is available you can't beat the economics of a polytunnel as you get so much for your money but be careful as to crops. I've gone from the modest to the largest single bay tunnel I could find and I'm still wanting more. I guess what I really want is something on the scale of the Eden project but a little bigger...