Upgraded myself from 10 Pros to S12, the difference is quite big gotta say.

I have the S12 and jave been looking at fiio fornsome time. How is the comfort for long term use? Would they be gym friendly?

Both. Comfort, or better to say the grip, and treble is a bit lower.

Both them few months ago, still my daily driver. The only thing I did was replacing stock tips for spring tips, and gtg.