Amir Johnson in a run away. He has more upside, is bigger, and has freak athleticisim. Maxiell will always only be that energy off the bench guy, while Johnson can improve to be that huge stat starter. Not only that but Amir is much more complete then Maxiell offensively. Maxiell can get you maybe 12 points per game if he starts, off maybe five dunks and a lucky shot.
I'd take Amir for his potential. He could end up being a starter for the Pistons. Maxiell on the other hand might have a role player type career, which is definitely not a bad thing considering what he does for them, but Amir has too much upside.