They bring to mind different things to me. "Physics" probably means something like quantum mechanics, while mathematical-physics probably means something like Chern-Simons Theory or the 2-dimensional Ising model. Basically "Physics" means questions that are actually of interest to physicists, while "mathematical physics" is usually about mathematical results which are inspired by (but don't directly involve) physics. But given the existence of MO, I doubt there will be many "mathematical-physics" questions here that aren't "physics" and so there's probably no harm in merging. At MO I'd be against merging.