Definitely Daffy. Besides being far more intelligent (rabbit tricks aside), his command of the English language is much more eloquent than Donald's. But if we take Kingdom Hearts as canon, Donald does have more battle experience than Daffy. So if it's strictly rooted in the cartoons, then I'd say Daffy, but if it's incorporating the video game Donald, then he'd win in a heartbeat with a well-placed Thundaga!