McD might lose some carries early to Fargas, but that will end quickly unless he gets fumblitis or something to get him in the doghouse. As far as being a #1 RB this year, I highly doubt it. As I said, he'll probably lose some carries early to Fargas and it wouldn't surprise me to see Bush vulture some goal line tds from him this year. Could he be a 1000 yd back this year? Definite possibility, but far from a certainty and if he ends up with 1000 and 7 tds (a good year for a rookie on a bad team), I'd hardly call that a #1 RB.
The only drawback I've seen with Stewart is the obvious toe injury, but you have to give the guy props when he runs a clean 4.45 WITH the toe injured. As long as it doesn't become a lingering problem, I would probably lean towards him over Mendenhall. I really like Mendenhall, but I have to think there was a reason he couldn't beat out his predecessor his first three years in Illinois. I also don't see FWP going anywhere for a while, so Mendenhall could face a platoon situation for a couple of years and Pitt did absolutely nothing to fortify it's leaky O-line this off season. Carolina on the other hand added a good combination of vet and rookie talent on the front line. Fox likes a smashmouth running game and DeAngelo doesn't fit the bill, so I think he'll just be used as a change of pace back.
Having said all that, I would have been happy with either and had I intentionally tanked games at the end of last season like some of my opponents, I probably would have one of them.