I think polytunnels are quite a bit cheaper than greenhouses.
In Winter a greenhouse would be lined with bubble wrap to keep the heat in, so it would not necessarily need to be kept heated.
We have a greenhouse, and do not keep it heated throughout the Winter. We have a heated mat (which we call the electric blanket), and a soil warming cable with hoops over it to drape fleece over to keep the heat in, which OH (aka Heath Robinson) made. It may have been cobbled together, but it works perfectly. ;D
Given the choice, I would always go for greenhouse.
valmarg