You want one with a G force sensor so that it KNOWS when something happened. If you're in a serious accident you may not be able to go stop it before it rolls over and erases the content.

The problem I have is that all of the ones I've seen with loop recording are built for cars, and none are waterproof. Without a waterproof camera, I'll have to be mounting it, unmounting, etc. Also the odds of being in an accident are probably higher in adverse weather so going with a non-waterproof camera is probably not good unless you just don't ride in adverse conditions.