There is no set law that one makes power and one doesnt, but interference do tend to generally have more power due to reasons allready stated. But thats not to say that all of them make more power.
Timing belts are one of those things that isnt allways a given, you can run the hell out of a car and the timing belt is offen last as long as a car that was pampered. Belts also usually show no signs (other than visual... but they are not visable on most cars) of wear untill they break. Chains will start to get slack... but are noisy and harder to replace. If your timing belt breaks its more than likely time to replace the valve seals anyway.... (gotta have some possative outlook on everything... )