I don't know if Jackson hit harder. P4P, easily... But McClellan could of literally fought as a lightheavyweight without a problem where as Jackson was a junior middleweight.
McClellan was a great puncher, but Jackson is in a class by himself as a one of a kind one shot killer.
Jackson was a bigger one punch hitter, at least P4P. At MW they are pretty close, both were monstrous punchers. G-Man had the better chin though, and his punches really effected Jackson when they landed, which was always the deciding factor in their fights. Neither were very good technically, in fact Jackson had the better guard.
Their first fight was extremely close, too. Jackson was doing well until a nasty headbutt, then he just stopped throwing completely.
Jackson by far. Julian was the smaller man with the weaker chin, and he still gave G-man a run for his money in the first fight due to his incredible power.