PPR League, definitely Bush.
Gore's numbers are about to fall off a cliff, and I'm a 49ers fan. They didn't pickup all these RBs for depth, they plan to use them. They systematically preserved Gore for the post season last year and it paid off, he ran for 5.6 yards a carry in the playoffs. They will do it again, Jacobs will vulture and James will sap his PPR value (what little he had left after 6 drops on 17 receptions last year). Kendall Hunter will rotate in too. I expect 200-220 carries and like 10 catches, he might get 1000 all purpose but I want nothing to do with him in fantasy right now.
Reggie Bush was a beast down the stretch last year and is a dynamic receiver, they are going to feature him whether it's splitting him out wide or whatever, with Marshall gone he is their best playmaker. The only thing I worry about with Reggie Bush is can he stay healthy, and I have my doubts, but to me he is easily the better pick. Especially in PPR.