I absolutely love both Petty and Springsteen, and (imo) they have two of the greatest backup bands in rock history. I think Springsteen has had more misses than Petty, but when he kills it, he's better than Petty's best.
So i vote Springsteen.
As for Bach v. Mozart, that's a harder discussion, largely because they wrote during different eras. If it hadn't been for Bach, Mozart couldn't have been what he was.
If we disregard that, though, and look at their music without that prejudice:
Bach was far more prolific (he lived a lot longer) and his music is much more interesting from a technical standpoint. He was also far more innovative.
Mozart's music is much easier to "feel," it's more emotionally accessible, and though less innovative from a technical standpoint, it was more revolutionary from a cultural standpoint.
I think Bach was more important, more influential, and superior in a lot of very significant ways. But i'm much more likely to put my Mozart cds on than my Bach cds, especially when Mrs. Mo is in the room.