Have had both, and have to say, so far this year I LOVE my poly. It is only small, but built to accomodate the space available, so therefore is wider than my old greenhouse.
Have at the mo Lettuce, peas, potaotes all ready, this never happened in the greenhouse.
Also have toms and aubs in already, cos of the width of the beds I can grow more, it is also better designed upwards so i have more headroom!
others will differ with me I am sure! ;D