I would absolutely roll with Mendenhall this week. The kid's a flat-out stud, but more importantly, look at the situational factors here.
1) They are playing OAK, who has given up the 3rd most points to fantasy RBs this season
2) Roethlisberger's 1st game back from the concussion . . . they are likely to want to take some of the pressure off him.
3) They are playing in PIT. This is not going to be pretty for the Oakland Raiders folks . . . not at all. I would be surprised if this game doesn't quickly devolve into a flat out rout. PIT will likely have the lead and just keep ramming Mendenhall down, through, and around the Oakland defense.
I have a few points of disagreement with my buddy ampant here.
1) I'm not so sure Mendenhall's a stud. If he is, though, Gore certainly is as well (he is anyway)--he has more fantasy points despite missing a couple games.
2) I don't think they're going to go easy with Ben. He practiced the week before week 12 and didn't play, and he's practiced all week this past week. The dude's ready to play, and I think Pittsburgh will have a full game plan in place.
3) I'm not saying Mendy's matchup isn't great, but as long as we're talking about matchups it's worth nothing that Gore went off for 207 yards and 2 scores on the ground 37 yards on 5 catches against the Seahawks in week 2. Even if you take away his 80-yard TD run, he still lit it up 22.4 points and averaged over 8ypc. So, if we're playing the matchup I'm not sure Gore doesn't have the edge.
4) I actually think the location of the game is a bad thing for Mendenhall. The weather in Pittsburgh will be cold and blustery (around freezing all day) and Mendy has had more trouble holding onto the rock than Gore (2 fumbles vs. 0). It'll be cold in Seattle, too, but take into account the fact that it snowed yesterday in Pittsburgh which will make the temperature at field level downright miserable.
So, yeah--roll with Gore. He's a stud, and he's your man this week.
Here's mine: http://www.fantasyfootballcafe.com/forums/posting.php?mode=quote&f=19&t=463620&p=3165802