The higher the base damage of your weapon, the more PS adds to damage. If you're 2shotting or 3shotting anyways despite theoretically increasing your PS, it's just better to increase ATH, since each level of ath is pretty noticeable. It's as if you decreased a whole tier in gear weight (~7.5kg). Having above average ath is a decent advantage compared to only having average ath.
WM is actually quite useful, too, and having >20wpf over an enemy is better than it appears (every millisecond counts), but a lot gets taken away from armor. For melee, the higher wpf you have above the average, the better; therefore, WM would help for you, even if the effective value only increases by 5-6 points. Most people are around 120-130, so 140-150 is quite good.
Testing STF builds for a while is the best thing you can do. That's what made me decide that I liked 8+ath and shy away from 21/21.