Your two favourites, I see? I'd pick Froch. I find him to be much better at 168, although both are overrated there.
I would actually pick Collins over Froch. For one, he was physically stronger than Froch. That alone is a problem, as Carl depended on being able to physically manhandle guys. In close, Steve would maul him. Moreover, at distance, Collins off-beat style and rhythm would also make him a lot harder to control with the jab than a plodder like Abraham. I would note that Collins also had a much higher ring IQ than a guy like Abraham.... or Taylor, or Pascal. Don't get me wrong, it's a close fight, but I really see Collins finishing stronger than Froch and outfighting him down the stretch.
I agree with this, I think it's the rhythm more than anything else which will allow Collins to bag enough rounds in a close one.
My thought process when picking Froch, is that his offkey sense of timing and that strange feinting game, makes Collins stutter before coming in. I think those moments of stuttering would only get worse after Froch lands some on him. Close fight for sure, though. Can see either pick.