Latest figures show the following residential market mix:
Polyester- 45%
Nylon- 29%
Triesta- 16%
Polypropylene - 8%
The mills are pushing PET and Poly products because they are cheaper to make, and they tout the "vibrant" colors and "improved" wear quality of both due to new production processes. Yes the colors are more vibrant. NO it doesn't wear a whole lot better then the old stuff. It still packs and abrades like mad.
The Treista fiber is a good bit better on the wear front and is very stain resistant. But, it's hard to remove some oil stains and depending on if you get the silk version or not, you can be opening up a whole new can of worms with vacuuming and cleaning problems. I'd stay away from it, personally.
Everybody knows to stay away from polypropylene.
To me, the colors on nylon are fine. It the PETs are more "vibrant", I can't tell it. And the wear of nylon is MUCH better than any other fiber except wool, and who can afford wool?
Nylon and Treista are hand and hand price wise for face weight these days, so you might as well get the nylon.