I much prefer the exile because of the fact that you choose what he/she is like. Revan comes with a history and as many said, you start out playing as your character, then turn into revan. I still think that the exile is more powerful because she has turned away from the force; she can live without it. She was the basis of Darth Nihilus' power and that of the Sith Assassins. THAT is power. Her ability to thrive off others. This could also be her downfall though.
Revan and the exile are also complete opposites, as Kreia says.
Revan = heart of the force
Exile = death of the force