If both players reach their potential, Green-Beckham is a far superior player and talent than Dennis Kelly.
That being said, due to his issues, Green-Beckham is the kind of guy who is out of the league in 2 years, and Dennis Kelly is the kind of guy who hangs around for 8-10 years as a backup.
I don't see much to lose from the Eagles standpoint in this trade, because Kelly is a guy that you are constantly looking to replace. If they can hit on Green-Beckham, they have a matchup nightmare on the perimeter. If they miss, they gave up a journeyman lineman.