Froch, simply because he's easier to hit. I have seen guys with better chins that Froch get KO'd. It happens.
Even when hide decline really begins, and he's losing fights, I don't see him getting ko'd. Like Hopkins.
Froch gets hit with about 5 times the number of punches Mayweather does. Mayweather's defences are so much better than Froch's. Both chins are top notch however. edit: Mayweather's chin probably hasn't been tested enough to describe it as top notch. It's certainly good enough for the job though.