AFAIK, this tent is made from nylon, the disadvantages of which you have described above and include sagging when wet. However nylon does have advantages and is used (again AFAIK) in expensive tents.
Polyester and nylon are both used in cheap tents, expensive tents, and everything between.
Polyester stretches less and is much less prone to UV damage. Nylon (at least before it's been excessively UV damaged...) is, all else being equal, stronger at the same weight. You choose, you lose.
Just as with other materials (e.g. frame building steel for bikes), there are different qualities of polyester and nylon fabrics available (both the base fabrics and the quality and utility of the coatings). To some extent you can get around the problems by throwing money at them, but if you try and get around the UV thing with nylon, you could throw the same money at polyester and it would still do better there.
Unless I was doing something like camping during polar summer for weeks at a time I don't think I'd be particularly worried about nylon vs. polyester: either one used by a manufacturer with sufficient Applied Clue should be fine for my purposes. I'm far more interested in the layout once pitched, and how easy it is to pitch and strike.