McClellan ranked very high at West Point and was a very capable General as far as organization and training an army. He was very meticulous at planning but was not a good battlefield general. Grant was not ranked very high at West Point but was relentless. Even after a defeat, he wouldn’t retreat North but regrouped and kept on pressing South unlike previous Northern Generals. The North had superiority in manpower and material and he used it to his advantage.
A great quote of Grant after he first took over the Army of the Potomac: “…“Oh, I am heartily tired of hearing about what Lee is going to do. Some of you always seem to think he is suddenly going to turn a double somersault, and land in our rear and on both of our flanks at the same time. Go back to your command, and try to think what we are going to do ourselves, instead of what Lee is going to do…”