Wow, have YOU opened a can of worms! From what I gather, the answer is "yes" they are all of different quality, and "no", they are pretty much the same.
I'm over my pay-grade here but different companies get their sheet vinyl essentially where ever they can get the patterns on them them need. Some of our more expert folks here will tell you that even "virgin vinyls" may have very different compositions. Generally, though, thicker is better. My liner is near the end of its life at 12 years, and I could probably get another 1-3 years out of it, but I am changing it because i have to rebuild the pool walls, so I have to change it. It's badly faded but it's still pretty thick though it's more brittle than it was.
A couple of years ago we had an interesting discussion on whether calcium in the water had ANY effect on the vinyl and it turned out that SOME have a higher calcium content than others (don't ask ME why) but we couldn't establish whether calcium in the water would prevent calcium leeching out of the vinyl the way it can from concrete or tile. It seems to not do that.
I personally think you should get the thickest liner you can afford, install it carefully, and maintain it carefully, for a long life.
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