First consider the location and foundation for your greenhouse. With regards to foundation you will want it to provide 1 - adequate anchorage so the GH Is protected from gusts of wind. 2- adequate drainage for the floor of the GH neither do you want to place it in an area where water accumulates in a heavy rain storm. 3- a foundation that has adequate insulation at the base to keep pests and weeds from creeping in. As for location you will want to keep it away from structures which will shade it part of the day. Once you choose your greenhouse kit see what type of foundation the kit recommends for it and proceed accordingly.
Now to your greenhouse itself. Will their be enough room to fit in potting and growing tables or racks and the other necessities which you will require? As far as your question of what material is the best for your greenhouse structure, I have no answer to that since the one I once had was the old glass and cement foundation one that came with the property I purchased. I am not familiar with the ones made from new construction materials at all.

I'd go for glass as it doesn't discolour with time like polycarb does. It is easy to clean too. My Hubby put a 6" wide 4" deep concrete foundation down then wooden battens that he screwed the greenhouse to. It has been in situ for 18yrs now without any problems.

If you can I'd put a row of pavers down the middle of the greenhouse [properly set so there is no wobble] so that you can reach both sides of the 'path'. You can either leave soil either side of the pavers and plant directly into the soil or use tomato pots or grow bags.
I have soil and I bury tom pots into the ground.

Good to have soil beds rather than growbags - much less watering. Polycarbonate can be good for the roof to avoid any broken glass falling in. But if you are buying new it should be fine for years anyway.
If you want to overwinter plants but not heat the greenhouse then you can stand pots on slabs of polystyrene, use fleece to wrap anything very tender. Keep plants on the dry side - but watch for drought too! If you water - do it in the morning, air greenhouse in milder weather and check for dead leaves/debris/moulds - keep it clean and they should be fine.

Having had a polycarbonate glazed GH I would avoid if possible. The channels quickly started to fill with moss in spite of having sealing strips. The window was very light and moved in the wind. I have just bought a new one for our new garden. This time have chosen toughened glass that comes in full sheets . Check out greenhousepeople website for special offers. Enjoy!

Glass every time for me. Far far stronger than anything else and it won't discolour, go brittle or blow away.

If you go for a 4 foot wide g/h, it will limit your available growing space by quite a bit. You'll need a 2 foot path which only gives you 2 foot for plants. Go for a 8 x 6 and you'll double your space....and believe me, once you get growing, NO greenhouse will ever be big enough !!!!

I would say that all had been said. Yes, glass, aluminium, un-shady aspect, and good foundations. I would say that Badfish's comment concerning the size is one of the most important considerations. The other would be, how much time do you have to use it if it was substantially larger and as I have found, have you got someone in mind to take over when you go on holidays?