Brad Pitt was in Seven, Fight Club and Snatch, so that settles it for me on a personal level. He's pretty much my model for masculinity. And for a lot of other guys my age as well- a fact I don't think should be underestimated if we're talking cultural impact.
Leonardo DiCaprio is pretty much my antithesis of masculinity- he's the man I never want to be. And he used to be that to a lot of other guys my age as well... it is only in resent years that the hipster shemales in the generation below mine has started talking him up as this genius artiste. Which he is, don't get me wrong. I just dislike him on a gut level.
All in all, I don't think there is much doubt that DiCaprio is more versatile, and just a plain better actor. He's consistently picked movies with mass appeal, while Pitt has gone for more niche roles.
The only thing that can be said for Pitt is that I think he's meant more to people than DiCaprio, and that his best movies has influenced society more.