I think Isaac has more upside and is more of a homerun threat, but I love the way Green runs the football. He isn't afraid of contact. Decent vision. Good speed (not great like Isaac). And my favorite, when you watch his film, look how quick his feet are. A lot of athletes are a bit rawer and have the long, deceptive stride. Long striders are often faster, but not necessarily better, particularly at RB. Quick feet help you cut and it gives you much better power (because of better balance and a better base).
Really like Green and I think Michigan would take him if they miss out on Isaac. I think Isaac is priority number 1 because he is something Michigan doesn't have close to. Isaac could be a homerun threat at RB or WR, but IMO would need more time to be college ready (I think Green will be ready when he steps on to whatever campus, Isaac will get snaps because of his breakaway threat, but not feature yet). Michigan has backs that bring a similar skill set as Green.
Only knocks on Green right now from what I've seen is that he has good speed, but not scary speed. And I wish he used his hands better (Smith uses his hands better than almost any back I've seen coming out of high school in a while, which is the difference between him being a 3-star and a mid-level, very good 4-star).
So, in conclusion, Green is my favorite back Michigan offered, but Isaac is probably better for Michigan because he brings something Michigan doesn't currently have. Both are great options though.
Film (can't embed): http://www.youtube.com/watch?v=TUC-Mu6Lswg