Campbell, and I don't think it's particularly close. Bush/McFadden are good receiving backs, and Schillens/DHB/Murphy/Higgins/Miller are solid options. Campbell would only need to have 462 pass attempts at a 6.5 Y/A average to hit 3,000. He's basically a lock to do that barring injury, whereas there's a decent chance that Leinart loses his job this season.
This is my line of thinking. If you tell me that both QB's play 16 games, then I'd say both would throw 3k. However, as sad as it is, Campbell is the more established NFL QB of the two and has proven he can play 16 games in a season. Who knows, maybe Leinart grows up this year and has a big year but there's also that chance the guy just doesn't have it, the Cards have seen enough and midseason they pull the chain and go another direction at QB. He's not reaching 3k if that happens so my vote goes to Campbell.