In my opinion, despite the fact that P:M is cool and all, it's a waste of time to seriously practice, assuming you have goals of success in Brawl, Melee, or any other game with an actual scene. Even if it's the "best" of the mods, the fact remains that it's still a mod, and while it might have some minor success at tournaments and whatever, I will be EXTREMELY surprised if it even gets close to the national success of the real smash games. Not to mention, with SSB4's most recent announcement, I think it's fair to expect it within the next 2-3 years, which leaves very little time for P:M to develop into a meaningful game, acknowledging the fact that game is still in beta.
Don't feel bad playing the game, it's cool and whatever, but to say "I want to be nationally recognized as good at a game. Which one should I practice, Brawl or P:M? I'll go with P:M!" is just sillly.