I like Benn on points. His prime at super middleweight was probably between 1992 and the McClellan fight in 1995. But at this time he was a superbly conditioned and well polished 12 round fighter, his workrate was excellent and his handspeed and power were superior to Froch's.
Froch is a little open for hooks and I don't think he outworks Benn over the distance. As for Froch stopping Benn, well Eubank (at supermiddleweight) and McClellan couldn't do it, and lets face it McClellan hit him with everything including the kitchen sink......so i wouldn't expect Froch to do it either.