Carter on form is better than Cooper on form. Carter off form is miles better than Cooper off form. Conclusion: Carter is better than Cooper.
I actually don't have anything against Cooper, and I do agree that he can be good on his good days. I just find it absurd that, given the choice, you would pick him over Carter.
As for their attacking abilities - I think people forget how good Carter is on attack because he's good at all other duties. As an example, Carter has 29 tries from 97 caps (one try every 3.34 matches), Cooper has 6 tries from 46 caps (one try every 7.67 matches). Sure, some of it is down to Carter playing for the team that scores more tries, but Carter's try-to-cap ratio is more than double that of Cooper. You can't argue that's not a mark of a better attacking player.