Physically, Dez all day. They are both physical wrs but Blackmon isn't big. That concerns me. He isn't terribly fast either. I don't think anything about Blackmon physically is going to be a problem for defenses. That doesn't mean that he can't be good though.
Mentally is not as easy. I'm not talking about off the field stuff. Route running, setting up the db, polish as a wr. I have heard that Blackmon coming out of college was considerably better than Dez was coming out in these areas. Weeden talked about how Blackmon knew how to set up the cb based on how the guys feet were set up. Thats pretty impressive for a college wr. I also heard a miked up Patrick Peterson laughing at how Dez only ran a few routes. If you think about how little he played in college, it can be expected that he would need polish. He is getting it now. :starspin
I don't think its fair to compare them as players today. If you were picking one guy for this year only, only a fool would pick Blackmon over Dez. Longterm, I would still pick Dez, but give Blackmon a chance he could surprise.