Yeah, it's an interesting match (Another character you could use in a sniper match would be
Azureyfear Winchell). It's a shame that LN matches, especially between two LN series, are destined to die, like Higashikawa Mamoru vs Touma or Touma vs Voldemort. I doubt that we can reach seven votes on this one or others.
Reiki definitely has the advantage in range (Looking at old Russian anti-tank rifles, Yuu's rifle is likely a modified
PTRD, so we can use that to establish a likely maximum range of 1km, about half of Reki's range). However, I don't think this advantage in range would necessarily be that useful in a dense urban environment such as Tokyo. The ricochetting technique sounds impressive, at what kind of ranges has she used that? Because if she can use it at long range it could be a good advantage to balance Yuu's paralysis technique. About dodging bullets, certainly positioning is key in a sniper duel, and whoever shoots first will give away her position to the other and be forced to relocate if the enemy is not killed right there and then. However, while Reki is limited to human movement, Yuu is not, and can relocate faster and more efficiently. She can also pull the trigger faster, which could decide the battle if they both find each other at the same time, but this would be up to chance.
I agree with you that Reki will definitely recognize Yuu's sniper rifle on sight or by sound. However, I don't agree with you on Yuu's Nopperabou ability. As it is described on the novel, it sounds like a powerful enchantment that's qucikly applied on the enemy she's sniping, and beyond getting a specialist on Japanese Youkai and/or folklore I don't see a way for Reki to counter it. Without previous knowledge of each other I also don't see why Reki would suspect anything supernatural out of Yuu's sniper rifle. I don't remember much about Hidan no Aria's anime and I only read like two novels before dropping it, but it seemed like the supernatural is even less common in that verse than in Zashiki Warashi, with most armed detectives being normal humans.
While it's not enough to change the vote, depending on how good the ricocheting technique's range is, my opinion would change from Yuu winning 8/10 times to 6/10. It would be a close match, but as I mentioned IMO the range advantage would be lessened in this kind of environment, while Yuu's superhuman capabilities would lend well to it.