And be careful to compare "apples to apples". Ask to see the product and for full details of the process to be used, insulating to be done, and quality of the windows or doors themselves.
Will windows be "retro-fitted" using the existing frames or will it be a "frame-out" installation. ("Frame-out" is recommended in most cases to ensure a good seal and the opportunity to properly insulate, for the greatest energy savings.)
Consider delivery timelines, length of install, and if there are any potential
extra costs and be sure to get them in writing.
Including all warranties and service guarantees and be sure to get them in writing.
Always ask for references preferably for most recent projects and be sure to follow up on them.
Be sure you hire professional installers, who specialize in windows and doors. Don't leave it to a "handy man" or another trade. After all - you wouldn't let your mechanic cut your hair would you?
They will tell you what you want to hear, not what you need to hear. This can be a large investment. You need to do it only once and properly. Everything that is done should be what is best for you- not anyone else. If it sounds like a gimmick- it probably is.
If you are uncomfortable with the thought of spending a lot of money, all at once with a company - just have them do 1 window as a test first. This will demonstrate how they operate and give you some peace of mind, before you have them proceed with doing a large job.