I think ultimately it boils down to whoever can afford the best lawyer and has the time to spend years fighting it in court. Even if somebody rips off your music you still have to legally prove it in front of a jury.
That being said though, the best way to copyright your music is to actually legally copyright it. Even if it does cost you, it's better than mailing your music to yourself or whatever and just hoping for the best.
Personally, I legally copyright everything I post here... plus I've got dated proof of when it was posted online in several different countries. But if somebody from some third world country decided to rip off one of my leads or riffs, there's really dick all I could do about it except publicly embarrass the guy over the internet for being a thief.
If it's just similar or has one or two catchy riffs in it that sound like something you wrote, you're pretty much screwed.