I dont know if want cooper over bryant, because they have different strengths and play in different offenses.
but Cooper is really good, hard to cover, quicker feet. Bryant when healthy and used is a monster.
Dez makes it difficult on the QB. They have to force the ball to him due to lack of separation. Some of that is a speed issue but most of it is a route running problem. Witten can still get separation and he has no speed.
He's good at scoring TDs but he's often not "QB friendly".
From the article in the link below: Dez Bryant averaged 1.85 yards of separation at target. He ranked 54th out of 55 receivers to see 50 or more targets out wide.