Ryan Mathews played in just 12 games last season, starting only 9 of them. Still, he finished with roughly 800 total yards and 7 TDs. Ronnie Brown played all 16 games for the Dolphins, starting all of them, and ended the year with roughly 950 total yards and 5 TDs.
That's comparable, right? The difference in yardage is made up for by the touchdowns, I think. The big difference is that Mathews had 42 less carries (158) than Brown did in 2010 (200 for Ronnie), and therefore a better average on yards per carry (4.3 YPC for Mathews, 3.7 YPC for Brown). I'll give the slight edge to the Chargers' RB, mostly because 2nd year RBs are typically improved over their rookie season.
Mike Tolbert, despite playing in all but one Chargers game, started only 4 games for the team last season. Yeah, I don't get it either. The NFL system for determining games started really needs to be fixed.
Anyway, Mike ended with about 950 total yards and and 11 TDs. "Shady" McCoy played in 15 games, starting 13, and finished with roughly 1,600 yards and 9 TDs. At this point, I'd give the slight edge to McCoy for having almost double Tolbert's rushing yards. A higher YPC average for McCoy (5.2 YPC against Tolbert's 4.0 YPC) backs up his spot as the superior back.
Winner: Eagles. The Chargers running attack is still based mostly on potential because at least half of the carries will be in the hands of Ryan Mathews (in theory). If he can improve over his rookie season, and he probably will, the Chargers could be the winners here. However, until that happens it's impossible to compare that potential to what McCoy and Brown have done in their respective careers.