Honestly, I think Evans would be an incredible deep threat for us even though he doesn't have elite speed. You can send him down the field and there are very few CB's in the league who could cover him without help over the top. He made Manziel look really good by just out jumping the defense and coming up with almost every contested ball in college. He'd also be unbelievable in the red zone with his height, arm length, vertical, and ability to high point the ball. Evans will be a better red zone threat than anyone currently on our roster and that can make a difference as we've seen the past 2 seasons.
Cooks is faster, but Evans' jump ball ability makes him almost as big of a downfield threat and a much better red zone threat. He's also a much better blocker which matters in this offense.
If somehow both of these guys were available at 30 or we traded up and got a choice, I would take Evans and pick up a slot receiver and/or returner later. Cooks fills a bigger need (slot receiver), but I wouldn't draft strictly for need in the first round.