YPC is about the best option you've got for comparing backs in the cases where there are two good RB's on one team or zero good RB's on one team. I understand using carries so to not be unfair to running backs on bad offenses where they are the only thing opposing defenses have to stop, but carries is unfair to guys who made the most of limited or shared opportunities. If you've got to go by just one stat I think its got to be yards.