I have the EZ pad, and so far it has held up fine, and that includes a Minnesota winter. It doesn't suprise me that the generic guy says that there is no difference, what else is he going to say?
I think that EZ pad is made by the same company that makes Rhino Liner spray on truck bedliners.
You can find out for sure how an EZ pad is constructed, and what kind of warranty is offered, by checking out their website.
Your generic pad could be just as good, or even better, or it could be a lot worse. I'd make sure it was of good quality- if the pad fails, moving the tub to replace it will be a major pita.
When you add up all the $$$$ you need to get a spa set up and running, is a couple hundred dollars here or there really that big of a deal?
Brewman