Cobb is as good as Jennings right now and is still only as old as Jennings was when he entered the league.
Their first two years look awfully alike stat-wise based solely on Rec/yds but Cobb runs the ball and returns. He's even thrown passes.
If anyone had a choice between taking a 3rd year Jennings or a 3rd year Cobb it would be a landslide - Randall Cobb.