How to choose PVC double glazed windows

[waterman]

I have had several quotes for new pvc double glazed windows what i am looking for is some info on the plastic profile,i know it needs to be 70mm in depth,but what else do i need to be aware of because if it is fully reinforced does it really matter.usual sales patter stating that their profile is the best i-e dont use that company because they only use cheap plastic and bog standard K GLASS.your thoughts and advice would be most welcome.thanks.