I was playing Madden and I just remembered the Cowboys acquired Cobb this offseason, and of lesser importance, Witten.
I'm changing my vote, and here's why...
Bad QBs NEED guys that can give them check downs underneath. Prescott has shown he has some chops, but overall he's still in the pretty early stage of a decent QB. He has legs, which will help him, but he's not fast enough that a spy would be ineffective. I think he'll benefit from having a veteran, sure-handed TE. I don't know if that's witten.. I can't imagine the time off does any favors, and he seemed old when he retired the last time. But, if he can still feign a block and run a button hook, I think he'll be an asset.
Cobb is shifty, and a pretty good slot player. I always thought the Packers should have played him more out of the backfield more often, in split backs or full house or something. He probably shouldn't run it up the middle, but he's very good in space. If you can get him on some screens or wheel routes out of that package, that's a pretty valuable read.
More often he'll just be good as a secondary target in the horizontal routes. With Cooper running verticals, that might be enough to get a few first downs while waiting for a big play to open up.