Cox and Reyes are built the same. Both are very thick thru the hips and legs. Cox seems to be more explosive at the point of attack. Watching Reyes playing DT and NG he is not very sudden off the ball. He doesn't use his hands very well to disengage from the blocker. That just means that he is raw and can use lots of pro coaching to make him a better player.
Reyes could be a become a very good player if he works at it. I think what separates Cox is his agility and explosion from the LOS. Cox is so much faster on his first two steps and hand work is why he is going to be a high first rounder.