I think the differences between the players are quite distinct, but don't necessarily show that one is a better choice than the other.
Green plays more of a power game, delivering a punch to stun and knock back defenders initially. If he makes contact he wins. He also has good feet, but will get caught using bad technique at times.
Cooper uses good technique, squaring his lower body well to absorb defenders, but he doesn't have a good initial punch. He catches and walls off defenders rather than jolting them back. This works most of the time because he is able to eventually anchor with his lower body, but defenders can drive him back to the QB. I believe the better ones will be able to take advantage of the fact that they are able to get into his body by using power.
That's why I favor Green if he can stay healthy. I think the technique concerns can be cleaned up but I don't think the lack of a good punch/power can be. Maybe I'm totally wrong, though, and Cooper's method will work fine.