Aren't they pretty much interchangeable?
It depends on what you mean by that. They both mean "third-person singular indicative present tense," so in that sense they're interchangeable. The
-th ending used to be the norm (indeed, the only option). Then the
-s ending came along and eventually replaced it (and is now the only option, unless one is trying to sound archaic). I imagine there was a period of time when both were in use and were thus more realistically interchangeable.