It's really to hard at the moment to try and say which player will have a better career, we should at least wait and see what Maybin can do in A ball. Marte is a very good prospect that hasn't had the real breakthrough season yet always been pretty good considering his age. He showed improved plate discipline and should hit for nice power, yet his avg. remains lower then one would like.
Maybin is a very intriguing prospect on the other hand, he has power/speed/work ethic/ makeup that you look for in a prospect. Needless to say I'm really high on the kid, but I will hold my full reservation on him till he gets more time. It's questionable what his avg/plate discipline will be like at this time and he could take 2-3 years in the minors depending on his progress.
So what I'm saying is w/ Marte you know more of what to expect, around .270-.280 w/ 30 HR potential. Which overall is a pretty solid potential for a prospect at 3B and is hard to pass on for Maybin who is more of an unknown. That said I really like Maybin and think he has a pretty good shot at putting up better numbers (speed helps obviously), but you would have to take the risk and w/ prospects that is hard to do cause they all have a certain amount of risk.