Yes, U Value is important. The guarantee on the window profile, colour, handles, espags (locks) and friction hinges & the guarantee on the double glazed units.
If upstairs windows do they have restrictors and that the openings meet with fire regulations (changes about a mth ago).
Is there toughened glass to any windows where the glazing starts below 800mm off the ground and toughened to glazing below 1500mm in a door or door unit.
Is there steel in the window profile (very important - no way of knowing unless someone cuts a window)
Are the windows glazed internally or externally.
Always best to go on recommendations by someone that has actually used the supplier
Best of Luck