Rockstar has now gone on record saying that the version of Grand Theft Auto IV that comes in the box will be identical for the PS3 and Xbox 360. They say there are no advantages in owning the PS3 version over the 360 version.
Furthermore, they go on to say that, "But I guess if you want the complete experience with the episodes, then yeah, you should buy the 360, I suppose." They are referring to the exclusive downloadable content that Microsoft paid $50M for.
Who knows if this content will ever find its way to the PS3, but for now, you'll only be getting the extras on the 360. They will, however, cost Microsoft Next-Gen Points, so for $60 you get the same game on either system.