I'm in the rare position of having seen both of these and it's a difficult call.
If there's a clear difference in terms of what's seen on screen then Nick Marshall at least attempts to right some wrongs and learns a lesson, whereas Robbie's learned nothing and has actively made things worse. Within their worlds they both have power and both exploit it and others, Nick having the advantage of magic powers.
For me this comes down to the sheer weight of "douche-itude". Nick Marshall has had a life time of being a douche and by the end of the film has seen the error of his ways. Robbie is a young douche with a high douche-potential for the future. Might Robbie change his ways by the time he is Nick Marshall's age? In the film he, like Nick Marshall, also seems oblivious to other people's true opinion of him unless directly told (see angry Swayze and Baby's dad's realisation). In my opinion he's not going to stop being a douche unless he too gets magic powers from female cosmetics and electricity, so I vote Robbie - a Nick Marshall for the future.