Interesting question. So, it's actually why I like Horn, over Surtain. First, neither are bad players. Both are really good CBs but, Horn grades out better then does Surtain, especially for us. Think about Richard Sherman and then look at both of these two CBs. Which one fits that profile better? It's all about how Quinn wants to play Defense so yes, I agree 100% that it all of it depends on the Safety we have but, Quinn will not be drafting for what we have today. He will be drafting for what he sees as our Defensive scheme and personnel in the future.
OK, so if you look at both of these players, I think Surtain is probably the better man up cover corner. If you are going to play that kind of style, he's very good at covering his guy all over the field. But that's not the Seattle scheme. Seattle asks their corners to cover a limited area and then turn coverage over to the deep Safety in the role coverage scheme design. Basically, their corners use a shade step or slide step or step kick technique. I, personally like to get a bump off tight technique and create this advantage off the snap but in the case of Seattle, they do it with a shade step technique, which means that they basically create cushion buy lining up head up on a WR and just prior to snap, take a lateral step out. Basically they are creating spacing in order to get into coverage. So, they will then play a very physical down the field. Very handsy, lots of contact and just always in your face, until they have to give you up to the Safety coverage. This is Horn, 100%. That guy is exactly that player, in terms of just physical, always messing with you down field, doing every little thing to get in your jock and piss you off. Surtain, he's not that guy. He's a cover guy all over the field but that doesn't really fit the scheme. This is not a knock on Surtain, it's just a scheme thing and what you ask out of your players to make the scheme work.
Both are very similar in size, Horn is faster and quicker. Surtain is probably a better boundry CB, he uses his size a little better and he will not get beat badly, which means he's always in position to make a tackle but he isn't really that quick twitch, sudden, explosive reaction CB. Horn is a press man cover corner. Now, that may not be very popular with most teams but it would be in Seattle and you would think in Dallas, now.
So what fits you better? Surtain is not going to be that guy that makes you think of Deion. He only has 4 picks in three seasons. He's going to be Troy Vincent IMO. Horn, he's Richard Sherman with better speed and quicks.