Depends on a couple things:
1) Are we being held off the ground or on the ground
2) Are they pummeling.
If they are pummeling and you release a little after the pummel hit, you will always ground break. (speed of the pummel will have some affect, so pretty much Wolf can always ground release us if he pummels, but Meta Knight can't always force one). If you are being held off the ground and they do not pummel, you will always be forced into an air release. If you are being held on the ground and not being pummeled, if you input a jump, you will air release and if you do not, you will ground release.