Havok is a good physics system, and alot of new games for next gen hardware for PS3 and Xbox 360 are going to use it alot more.
Ageia is also a good soloution, and this is used in the Unreal 3 engine already, and the new Tom Clancy - Ghost Recon Advanced Warfighter.
I think the physics based sound feature is rather impressive. That an action will actually generate the sound, rather than it being pre-programmed to do so.
I don't know which one is more powerful to tell the truth, but I know that Ageias hardware could actually be built into GPU's, increasing the speed of processing of physics.
I do believe the Havok system is a bit more realistic though. I may be wrong, but it just seems to be, after watching the Ageia demo videos...