Personally I find Bucky to be FAR more dull a character now. When he's not being a killer assassin he's deadly boring to watch, at least in every post-CW appearance we've gotten so far. And hell even in the What If episode where he teams up with Red Guardian.
Sam can be pretty dull as well, but at least he's likable and knows how to crack a joke now and then. And he's also someone I care a lot more about frankly than Bucky. If Sam is nearly killed in a giant battle to the death with Red Hulk, I'm sure I'd feel that a lot more than if it was grim and moody Bucky.