Still under investigation, but what's certain, is that miners using old P2Pool versions, at this moment, are likely to have caused it. As for the causes and reasons... too technical for me, someone else might shed some light.
There is no "better" chain - all nodes can communicate with each other through the network, so i think payouts aren't affected. Just some more decentralization. (and a bit of confusion)
I'm pretty sure it is affecting payouts, though; the whole point of the share blockchain is to decide who gets payed. So people who mined before the split got paid for those shares twice, and people who mined after the split are only getting payed by whatever branch they're on.
But that's just my impression from the talk in this thread. I'm not a developer either.