True about the warranty, but the question remains; what truely makes one vinyl window better than another?
As mentioned the quality of the vinyl.
The dies used can vary greatly and will affect the windows structural performance.
Manufacturing equipment used by the factory.
Quality and experience of the employees performing manuf. tasks.
Management at the factory.
Honoring warranties and customer satisfaction.
Manufacturers and Installers reputation.
These are some of the items that wil also affect long term durability. Cheap windows will typically lose their initial energy efficiency over time. The question should be "what makes a cheap window Cheap"?
All Manufacturers define a market segment they are interested in capturing and then make a window designed to appeal to that segment. If someone purchases their furniture from a Discount Store, has Formica Counter Tops in their kitchen and is the type of person who only purchases anything when it's on "Sale", then it is likely they will have difficulty understanding what quality is. It is akin to looking at automobile tires and deciding they are all the same because they are all black, round and have tread on them.