George stayed true to his rock roots while McCartney has always tried to be socially relevant. Just look at his collaborations with Michael Jackson, Stevie Wonder and Kanye. Coming Up was his attempt at disco.
I think it was Bill Maher who had an interesting theory on their breakup was because Lennon was losing the battle for A side releases at the end.
Doesn't surprise me. McCartney saw himself as the leader of the Beatles (or at least, "all equal but Paul more equal than the others"), and in subsequent years the others made comments to the effect that Paul's overbearing, controlling attitude contributed to the breakup in a major way (e.g. the Maxwell's Silver Hammer recording sessions).
Granted that toward the end, Lennon was losing his way as he was increasingly devoured by Yoko's insanity, so the quality of his songwriting suffered. But it's hard to sort out which was cause vs. effect at this distance. I imagine it wasn't very inspiring to be in a band with such discord.
My theory is that a band pretty much has about 7 years to be relevant, before they turn into a nostalgia act, and the members start growing apart. Some hang on just because they need the money.
Basically, The Beatles broke up at precisely the right time, before The Law of Diminishing Returns started setting in. By 1970, a new generation had taken the reins.