Difficult one, most buy one to suit their pocket, usually Aluminium. You can get coloured but they cost more. My advice would be to think ahead, if you're confident that you'll get a lot of use from it, make sure it's big enough.

The biggest one that you can afford that will fit in the space you have. I agree with Heron aluminium is cheaper and survives fine. It really depends what you want to use the greenhouse for, flowers/vegetables/plants in pots in a plunge... If we knew more we could help more. Join GoY Guest and tell us what you plan to do with the greenhouse once you have it.

Check out the yellow advertiser newspaper as you could find a good inexpensive secondhand one. I picked mine up from there, 8' x 6', £35 disassembled ready to pick up with spare glass.
ps. no matter the size, it won't be big enough. ;-))