I think he was neither.
He was a perfectionist. Had circumstances ever been entirely to his liking, the perfect setting for a text book charge, he would sure have accomplished great things. But unfortunately that day never came. The fear of doing something less than perfect made him do less than necessary. But not because of lack of courage. Or maybe a lack of courage to make himself arguable or criticizable. We here have a saying "Only he who does not work at all makes no mistakes". Little Mac did not want to make mistakes.
Edit: Nevertheless Lee held him in great esteem! When asked who he thought was the finest Union general, he said "McClellan, by all odds!" He would never have said that about a traitor or a coward!
2nd edit: Too bad there was no "None" option! As I did not vote now I can't see what others voted for. To see it I would have to vote for either option, which would put my curiosity above McClellan's reputation - naw, I can't do that!