Well they really aren't THAT different, it's just that the SR forks are the originals and therefore are getting pretty rare. I wouldn't ride one though, the SenioRs are infinitely stronger than SR forks. It's just like something you'd have that you'd almost not want to ride so you wouldn't break it, like full core Protos.
As for the SMX, no I don't regret it at all. The SMX is the best fork I've ever ridden, period. It's relatively light, strong as hell, and the VertX threaded insert alone is enough to keep me riding it vs any other fork. Starnuts suck.