Having been born in Boston, I am a life-long and die-hard Red Sox fan. It's in my blood. Because the Marlins are a National League team, and I live in South Florida, I root for them unless they are playing against the Red Sox (they will play each other this Summer, and I will root for the Sox).
I believe that the Rays have a stronger fan-base, and I'll explain my non-scientific reasoning. South Florida, Miami-Dade county in particular, has too many diversions and activities that work against any strong interest in a baseball team. Even the Dolphins (football) have traditionally had a hard time selling tickets when they were actually playing well. I remember a time years ago when car dealerships would buy tons of tickets to a game just to avoid a television blackout of the game on any given Sunday. The three counties that provide the most potential for interest in a "local" baseball team like the Marlins (Miami-Dade, Broward, and Palm Beach) provide too many diversions that work against the great game of baseball. We live, in essence, in a huge playground. Consequently, there's plenty on which to focus beyond sports teams and the money or effort needed to remain truly devoted to them. Many South Floridians become Marlins' fans as soon as they reach the second game of the World Series. I believe that issues involving diverse nationalities also work against cultural interest in the game.
Anyhoo, count me in as a Marlins fan, although my devotion to them is faint at best. My wife is a huge Marlins fan, but she's delusional and calls my team the "Dead Sox". Weirdo.