Hi Ches, my apologies if any of my thoughts below don't help, that is perhaps because my circumstances are different to yours but I did read your question carefully, believe me. I have my veg garden at home, fairly large plot and also a need for bedding plants and baskets. I grow toms, cues, peppers and chillis as well as sweet potato, salad stuff, early courgettes, dwarf beans etc in the polytunnel. In fact I have two both 14 by 25 foot and because I have built on some of my garden recently and I am getting older and want to get ahead and behind the season and the weather I think my main food production will be in the tunnels. BUT BUT BUT and it is a big but I also like to start my own plants and bedding plants etc from seed and the tunnels are no use for this earliest part of the season. I am in process of reassembling my glass greenhouse (after the building work) and will re-instate my home made propagators (because I am at home I have power and security and can water and fuss about every day).The greenhouse is great for bringing plants on, starting pots and baskets and getting such stuff as beans and pumpkins and sweet corn ready safe from frost early than in open ground. Am I making sense
I guess I am saying polytunnels let you grow bigger stuff and let you start early and carry on late for many crops (I am also storing logs in one) but for early work on seedlings etc a greenhouse is needed.
Perhaps that is something you have already decided for your situation.
Enjoy.
R
p.s golden rule of greenhouses and polytunnels - it is never big enough!!!!!!!!!!!!!!!!!!
pps, You can't paint fences or cut up pallets and logs in the winter in a greenhouse but you can in a tunnel.