I'm not saying his story is bad, just that he is the "typical Rockstar fare" character - a troubled man who is trying to achieve a goal he deems important through being a pawn in everyone's court.
Frank Woods, on the other hand, is just another CoD hard guy, but, man, he is a very badass, emotional, likeable hard guy. He's the Soap McTavish of Black Ops - there are at least four instances of him being awesome.
I can't think of none myself for Marston. In fact, at one point, doesn't he, like, work for the Mexican Army against the rebels? Surely a great man.