I have a greenhouse. Personally, I think its nicer to look at ( its in my back garden, in line from my kitchen window) Also, I work in there, and like the fact that I can look outside - the only polytunnels I have seen have opaque or green covers.
I think a greenhouse is more practicable, as you get more hight to the edges, when a poly tunnel will be low at the sides. Latest greenhouses have safety glass, and you can get automatic opening ventilators, and various kinds of staging for standing pots on, of working at.
Wooden ones look more natural, but will need regular waterproofing.