Peterson's the rare back I'd make an age exception for, and even so, I'd want him on a fairly measured contract where he gets paid a lot each season, but his falling off a cliff doesn't kill your cap. He's a physical freak, though, and would be a huge upgrade to Murray. And I like Murray a lot, too.
That said, I think MIN would be crazy to cut him. They might, because they've got a stupid contract on him, but I'd expect them to adjust that in the offseason and get past his off-field problems if they possibly can.