From personal experience (very limited with the 31) I prefer the aesthetics of the 21 over the 31. Everything seems more uniform e.g. the distance between pivot-to-stop pin matches the machining hole-to-handle screw, and the clip aligning with the lockbar cutout. Ergonomically, apart from the clip the difference between the two is negligible, with the 21 having slightly better traction on the inlayed models due to the gap between the inlays. I didn't find the clip to be a hot spot on the 21, but apparently some do that's why they changed it.
The 31 feels slightly different when opening...like the detent ball is being sucked out rather than the crisp pop on the 21. Maybe that's due to the larger diameter of the ceramic ball. Disengaging the lock feels much smoother on the 31 though; similar to the Inkosi, it doesn't have to overcome the detent ball and because of that the action is very fluid.
For me, the perfect combination would be a large 21 paired with a small 31. Hopefully someone with more experience carrying the 31 can chime in~