Depending on where you live Sarah a polytunnel can be the difference as to growing veg, or not growing veg. I couldn't grow anything but brassicas without my tunnel, which is bursting with stuff at the moment. But as Nick says, sometimes its a combination of all types that you need. I got a greenhouse last year and this year it has meant me being significantly further ahead with seedlings than usual. But that said I still have also made my own Enviromesh cloches to cover the brassicas outside - had alot of problems with cabbage white last year.

Good luck and welcome to the Vine!