Ask about the profiles used:
While choosing windows it is very important to choose a quality profile system, the reputation and experience is very important. For PVC 5 and 7 chambers in profiles are the optimal number, chambers represent air pockets in the profile which have thermal and noise insulating functions, the number of chambers are made according to the thickness of the profile so if you have 10 chambers in a 60 mm thick profile you have absolutely no function. In aluminium profiles thermal breaks are the most important aspect. Feel free to ask your window company about certificates with profile performances.