Many times the physical differences between windows is very difficult to discern, unless one manufacturer is providing a unique feature. Obviously length of warranty and durability of finish are factors, but we usually advise our clients to select the windows which have the best service team in your area. Some windows may be great products, but if there are issues with adjustment or function, you want to make sure you can get someone to fix them and not have that be a hassle. Would suggest asking some contractors you trust what their experiences have been.