I'd take Johnson. Nothing against Froch but Johnson brings an iron clad chin and a wealth of experience. Plus for his age he seems to be still a very good pressure fighter. Better than he was when he fought Hopkins. Also the move down in weight should see his punches being a bit heavier. Froch is not hard to find..
I'd go with the Road Warrior. Johnson too tenacious, too much fortitude. But we won't see because Froch will get stiffened by King Artur.
Froch by late TKO imo don't overrate his win over alan green, Johnson is too old now to go in to the trenches with Froch. It's Alan Green he isn't very good even ward make him seem a bit better than he is, but I think ward was doing what he liked against green and had a relaxed night.