here's the major difference. if he is a lottery pick he makes millions. If he is a late 1st round pick he makes hundreds of thousands. If he is a second rounder, he is guarenteed nothing. That is a big risk to be on the bubble of a 1st rounder and 2nd rounder.
If he stays next year and blows up even more like Fizer did his JR. year, he makes a BUTT load more money. If he is smart, he buys insurance on himself incase of injury and works on the game so that he is a lottery pick and set for life.
He can ask Stinson about leaving early and how he is enjoying life. Also ask Fizer how the NBA game is - that guy dominated basketball in college and now is in the Middle East playing. Lucky he got the 8 million guarenteed of being a lottery pick.
Rookie Salary Scale in the NBA
http://www.hoopsworld.com/Story.asp?story_id=9301