Pay attention to Antoine Winfield's status for week 7 on Minnesota's D. He's the best tackling corner in the NFL and great against the run. He was out in the 1st quarter with a foot injury against Baltimore and Rice had a great day (especially receiving). If Winfield is out again, I would start Mendenhall.
That said, Lynch has produced since coming back in week 4, but hasn't had a breakout game yet. He's on a terrible offense, but Carolina is terrible against the run (and a terrible team, period), and if there is anything that Buffalo can do adequately, it is run the ball (note, I said adequately). So, he's worth a start, especially in PPR as he's a check-down back in an offense that checks it down a lot.
Portis is up against a strong D, and while he may see 20 carries, what is going to do with all those carries? Probably not reach the endzone...probably not do more than 2-3 yards per carry meaning 40-60 yards. The Redskins are dealing with a new play-caller and are dealing with the Jason Campell opera right now. I don't think you can expect anything more than what he's given you so far after 6 weeks of play. You'll get points out of Portis because of all the carries, but not very many.
So, if Winfield is back, I'd keep Mendy on the bench and go with Lynch. If Winfield is out, I'd go with Mendenhall. Mendenhall has more touchdowns and more fantasy points in less starts than Portis, and I would classify Portis as "bust-worthy" at this point. Lynch still has plenty of upside as he works himself back into game shape. I expect a decent outing from Lynch this week, somewhere in the 8-12 point range in standard scoring, and a bit more in PPR. Portis will get you 4-8 points IMO, and Mendy has the potential for at least as much as those two, probably more if Winfield is still out.