This might be somewhat controversial but I think Cooper is a better overall WR than Dez in his prime. Dez was way more dominant athletically but his game was very basic and lacked refinement. He was unstoppable on jump balls/fades/slants, and Romo knew how to maximize that ability, but once that peak athleticism was gone so was his ability to dominate. He had a higher ceiling than a guy like Cooper but he never developed the rest of his game and so his peak was short lived. He was a better red zone threat than Amari but other than that I'm going Amari.
I don't think that's controversial at all.
Cooper was the fricking fourth overall pick.
Granted, Dez was drafted at 24th or whatever and it was way lower than he should have been drafted because of off the field stuff, but even if he was squeaky clean, he's merely a similar top-10 talent. I think you could easily argue that Cooper was the better prospect coming out, which is why he went fourth overall and why Dez was more of a 5-10 type.
As for what they became, Dez was great and part of me says if he wants to come back on a minimal deal, let's load it up and let him come be our 4th WR, but his entire career he distracted not so much the team, but I think himself. He was constantly having minor screw ups. The finger touching the out-of-bounds on the Hail Mary that would have beaten the Giants. The "no catch" incident in the playoffs against Green Bay: yes, we now know that it was a catch, but Dez, is it so hard to just secure the ball and fall down? It seems like the guy was always making the easy look slightly more difficult than it had to be. Yes, he was capable of amazing physical feats, but it was also "always something" - a drop, a bad route, a fumble, a double-clutch on what should have been a clean catch, etc.
Dez will go down as one of the Cowboys greats, but he also was a bone head who simply did not reach his talent peak because he wasn't smart enough or focused enough.
Cooper has all the same talent, maybe more. He could easily eclipse Dez.