Green all the way. Alexander "may" be the better back, but you can't ignore strength of schedule. Some people argue that Holmes will have a better year than either of them ... but would you want to pass Green to an opponent? I wouldn't.
In any game that Green Bay's receivers are healthy (and granted that may be the odd game), the defense will be spread so thin that Green will get 10 yards before he even starts running. I just see a win/win scenario for Green right now. If the WRs are healthy, they will spread the defense. If they're not healthy, the workload will fall on Green. Either way it looks to be a good situation.