30 is smaller, easier to carry, built for CC but takes only 10+1 compared to the full sized 21 which can take 13+1. (I live in CA so magazine capacity isn't even a factor. We're limited to 10 max.)
Depending on your hand size and personal comfort, the fuller length of the 21's grip might likely give you more control.
My favorite answer is "both." Currently I own 21SF which feels so nice -- in my hand and to shoot. If/when I consider CC, I'll probably add a 30SF to my roster because I'd be able to share the same ammo between the two.