You want to know why Monroe is better than Kanter? Because he has played in the NBA. Everyone thought Kwame Brown (1st Overall Pick) was the next KG. No he wasn't. Monroe's last 20 games or so have been great, 12 points and 9-10 rebounds is great in only 27-30 minutes of action especially when he's only a rookie.
There's no question Greg Monroe has put up solid numbers over the past year. He will IMO continue to be a solid PF/C in the NBA.
Having said that, I would pick Kanter because he appears have more upside with his offensive arsenal.
He has the size and length to be a big in the NBA. On top of that he can score inside, he can stretch the floor with a 3pt range, and can get to the line and make foul shots.
Monroe is a safe pick IMO - we already have someone similar in Favors.
But if you want a potential game winner of a player, that rare someone who can hit that shot with 5 seconds left in the game? I'd have to go with Kanter.