I am impressed with Gilbert's athleticism and ability to make teams pay for bad decisions or errant throws, but he gives up way too much in between. I do not think he is a great cover guy, just very opportunistic. Still...would you rather have a guy who allows 0 points or a guy who can score 7?
Dennard? He can lock down a receiver. Flat out. If you put on a game tape with no context to what you are watching, you wouldn't know who was running the route and who was covering because he is that smooth, decisive, and efficient in his coverage and smothering of receivers. He will make the occasional INT, but not as much of a threat to get the pick 6 at any time. Often it is because he forces the QB to throw it where neither he nor the receiver can get it.
Me? I'm taking Dennard hands down. I don't even know if I would want Gilbert. I think he will be exposed by NFL QBs who will make less mistakes and offenses with better receivers running better and more complex routes.
I see Dennard as a lock down CB, perennial pro-bowler. I think Gilbert will be a bust as a 1st rounder. I think he will make headlines and highlights here and there, but I think he will also become a liability.