Brad Pitt has more versatility in his roles, he's been the sex object, he's done the action film, and, more recently, he's been taking on [a lot] of three-dimensional, harder studies. Legends of the Fall, The Curious Case of Benjamin Button, The Assassination of Jesse James by the Coward Robert Ford, etc. I think he has more star power, has had more coverage, and we've been able to see him in more roles.
But I think Leo is a better actor, and he chooses his roles much more carefully, and doesn't really go out of his Leo-Box. However, what he does choose to be a part of, he puts 1000 percent in it. Everything I've seen that has him cast is automatically one of my favorite movies. Blood Diamond, The Departed, Inception, Romeo + Juliet [although it was Bhaz Lurman, so a little strange], Titanic [which is such a better movie now that "My Heart Will Go On" isn't playing on every radio station every five minutes] Revolutionary Road, etcetcetc.
So I think that at first glance, Pitt seems the better actor, but you have to be more of a movie "film" lover in order to appreciate his films.
As for Tom Cruise..... Why is he even on this list???