My take on the Surtain vs Horn debate
Athletically:
Horn is a slightly better athlete, both tested well but Horn edged in most areas on his pro-day with neither doing a 3-cone, I think Horn looks on tape to have slight better change of direction but that's probably a knock I have on both of them.. I don't think you can be too negative about Surtain results though, even pro-day adjusted they are still very good numbers.
Role:
Horn following the best receiver was impressive. He always took the biggest threat, regularly popping up all over the place. It meant he often had tougher matchups than Surtain, and saw more targets. Surtain generally spent a game playing one side, and was rarely targeted. That's a testament to the respect Surtain drew, but made watching his tape a snooze fest in comparison.
Technique:
I think Surtain is more consistent with his hand use but both are elite in this area. Surtain IMO does a better job locating the ball, and high pointing on deep routes, while Horn is better at breaking on the ball and undercutting the route. Both look a little stiff on the back pedal, and sit a little high changing direction which is common for tall guys, Surtain a little worse than Horn. On run downs, both are disciplined containing the edge, fighting off blocks, but Horn is more likely to make a business decision on the tackle with Surtain more comfortable making the tackle. Surtain is a good tackler.
Splash plays:
I wouldn't send either on a blitz, and I highly doubt Horn will force fumbles with his mentality whereas Surtain has and will. I think Surtain will get picks high pointing the ball while Horn will get some undercutting routes coming out about the same but I wouldn't expect either to get as many as Diggs will over the course of their career. Neither look to have the natural hands Diggs has, but both look better athletes and are better technicians coming out of college.
NFL transition:
I expect Surtain will do a better job hitting the ground running. He is a boundary corner, that knows his limitations and will slot right in on the right side and do his job well. I think Horn will need to get his ego checked, and learn to be less aggressive undercutting routes. Horn has already faced good opposition so the transition won't be too big for him and eventually he's the type of guy you want to follow Julio Jones or Amari Cooper. Surtain will never be able to do that. I think the floor for Surtain is Brandon Carr but is a good chance to be a top corner, plus run defender, that if protected against super twitch guys regularly makes the pro-bowl.
The floor for Horn is burnt toast that never learns he can't be so aggressive undercut routes, and has to tackle, getting ran out of the NFL. That's a bit of an exaggeration as he's already shown competent against NFL level talent I still think NFL receivers and more importantly QB's are simply better at exploiting mistakes. Surtain does grab a bit too but because he's less aggressive, and rarely gets caught behind, it doesn't stand out as much.
Assuming Horn adjusts his game, he could be a unique player in the NFL that can follow Julio Jones one week, blanketing Travis Kelse the next, and then lock down Odell Beckham. Personally, I think it's a tough call between the two but I think I like Horn's unique skills matched up with Diggs, a little better. I think we have a small speed issue with a Diggs/Surtain partnership and might struggle with big slot guys. Horn being that little bit quicker, and ability to move inside might be more complimentary.