Is the warranty transferrable?
Will the company be around in a decade?
Will you save all the paperwork? Most people don't.
What type of window? Vinyl, wood, etc? Vinyl is white and will fall apart but what are the warranting? Glass seal, air leaks, paint failure, etc?
Do they still sell parts for 10-20 year old windows? How about 50-100 year old windows?
Those are the questions I would think about.