Direct answer: compare complete quotes, not just prices
A quote must include window, glass, removal, installation, sealing, shutters, trims and finishes. Otherwise, the lowest price may not be the real final cost.
Not defining priorities
Noise, heat, cold, security and design require different decisions.
Ignoring glass, shutter box and installation
The frame alone does not define performance. Glass, shutter box and installation are critical.
Using the same solution everywhere
Each room may need a different balance of noise reduction, solar control, ventilation or security.
Mistakes before accepting
No technical visit.
No final measurement.
Unclear warranties.
Unclear removal and finishing.
Practical recommendation
Choose the quote that explains the full solution most clearly.