The only thing I'd say about plastic is that on my Norfolk greenhouse, the sections are not secured anywhere near strong enough and have a tendency to blow out of their plastic runners during high winds.
In hindsight, I would have gone straight for my aluminium standard one and not bothered with the Norfolk which is now held together with various bits of washing line, uv resistant tape and the odd panel cut out from an old walk in g/h.
Hope that helps !
