24 mixups are super predictable (KL always grabs his hat for the mid), only lead to spin half the time (depending on stance, but always in corner), has bad block advantage (only 24 1+3 is safe on block, but it pushes you to fullscreen), and doesnt build nearly as much meter. Plus, 21 spin into combo does the same damage as 24 spin into combo. Sooooo i would say that 21 is better most of the time, as on block you are still safe, 21 always leads to combos, you get massive chip and meter building blah blah blah. But 24 is nice to use in the corner as you will always land that spin no matter what stance you are in.
Thats my take, play around and make your own opinions.