Carpet pile density is very important when buying carpet. I have seen lesser carpet fibers (polyester) perform well when they are tightly packing into the carpet backing.

Smartstrand is a good fiber, but Mohawk is now regularly blending polyester into their Smartstrand products. Some products have a 50/50 blend or a 75/25 blend.

The Smartstrand line is like everything else - there are budget, entry level versions with very low face weights, low density and low twist; and there are really good ones.

Good twist, a rating of 5 or better along with density as high as possible is what I would look for.

A way to make carpets feel soft is to lower the density. Higher density carpets are firmer.