Personally I think Southpaws have a big advantage since they're pretty rare in boxing, from my knowledge anyway, and simply because of that most people aren't used to the difference in reach and stance simply because they're a southpaw. I myself am lucky enough to be able to switch between Orthodox and Southpaw stance as I see fit, even though I am an Orthodox boxer, it's a nice trick to have to catch them off guard.

A disadvantage would be that you would be mostly fighting Orthodox boxers, so meeting a Southpaw like yourself, could probably confuse yourself. Other than that, just develop your own style over time and it will be fine, don't worry about being Southpaw or Orthodox.