I need a QB: I have been offered Phillip Rivers for Rashard Mendenhall, I also have LeSean McCoy and Matt Fort?

Right now my QBs are Matt Helbeck and Brett Favre, I see no future with either one of these old men. I have three great RBs, but we only start 2 RB in our PPR league (no flex). I have been offered Philip Rivers for Mendenhall, the other owner has indicated he would trade Rivers straight up for McCoy also. I've countered by offering Matt Forte straight up. Which RB would you trade for Rivers? I feel like Mendenhall has the greatest potential out of the three - McCoy and Forte are sort of interchangeable to me, though I think McCoy will get more rushing yards.