If you need immediate help, I'd say Bush. If you are looking more long-term or able to incur a bit more risk, then I'd say Sproles.
I think it is safe to assume that Fargas will be out for a few weeks. And with DMAC being seen in a protective boot it would be reasonable to assume that he will at least be limited and possibly held out for a game or so. Regardless of whether or not or even when Fargas returns, DMAC will be getting significant playing time this season. Bush's summary: he is probably going to get a good amount of touches this week and maybe even the next. I believe OAK has a bye in week 5 which means 3 weeks for Fargas and DMAC (if he is even that injured) to get healthy. Therefore I believe Bush has limited value beyond that point.
Sproles on the other hand is a bit more of a risk IMO but also has the potential to have mor long-term upside. Nobody knows the true extent of LT's injury and therefore nobody knows how much LT/Sproles will play relative to each other. Couple that with the fact that SD plays on Monday and you have a risky proposition this week. However, knowing the type of injury that LT has (turf-toe can linger for a very long time until you get ample rest) I think that the possibility exists that either (A) for at LEAST a portion of the season LT is not able to "carry the load" and thus Sproles gets a decent # of touches. Or (B) at some point LT gets shut down for a while so that he can heal and Sproles significant playing time.
Caveat Emptor ~ The thoughts and opinions expressed in this reply are simply that: thoughts and opinions!