What caused the chain to split? Is there any reason why the new chain would be better than the old one?
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)
My understanding from previous posts is that the old code was prone to splitting the share blockchain due to some bug. A fix was pushed to GIT, but some incompatibility between the fixed and unfixed versions meant that the share blockchain split one more time and then stuck that way.
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.