I have a couple of dozen western hats that are 20 years old and newer in a mix of Resistol, Stetson, and Bailey. All are 20X or better. When I have done apples to apples comparison there is little difference. The older Stetsons made in St Joseph MO may have had an advantage in felt finish (exception would be the resistol 20X "Black Gold", more on this...) and used a finer feeling leather for sweatband but the sweatband stitching was coarse and would irritate my forehead. In other words, the Stetson's felt better to handle but Resistol's were more comfortable to wear all day long, day in day out. Both Stetson and Resistol are not colorfast in black or dark brown if rained on.
All of the Stetson and Resistol hats I have looked at in the last two years were indistinguishable from each other exclusive of liners and trim. The Stetson's have the Resistol sweatband style now.
Resistol came out with a 20x about 25 years ago. This hat had a much better finish and fitted a felt band with a tooled buckle set and it was pounced to have the silkiest feel. This is their Black Gold hat. I have a few of these and they compare in quality, regardless of the beaver content, very well with 100% beaver hats.
If you go back to the 70's and earlier the game is quite different as with fedoras and other men's dress hats. The single digit "X" hats are tough, durable, and have a great feel to the felt. Newer hats in the single digits seem to all have some wool in their felt or at least some very coarse hairs of some origin. The newer hats do not stand up to being soaked (you know the usual stop at a watering hole, take off hat, fill with water and let horse drink out of it activity...).