luffy's a bit greedier and more aware of his selfishness. he acknowledges when he's being selfish and thanks people for putting up with him and he's stated outright that he'd rather have all the meat to himself than be a hero "who shares his food with everyone. Goku takes from people and doesn't understand that it's a problem.
Goku also seems easier to fool in combat. I can never help laughing when he exclaims "but that's not fair!" whenever an opponent in a life or death duel pulls an underhanded trick the first time. but luffy's often been heard to say that sneaky moves in combat are to be expected, even if he himself prefers to be straightforward.
plus, luffys not a fighting junkie.