Either would be an upgrade, if you have the money for the Razors then consider the Meopta and Swaros as well in 15s. I have Meopta's and they are great optically, probably better at 15s than my Razors are at 10s, I have never looked through a pair of UHD Razors but I'm sure they are great, they are also big. If your set on 18's and have the money the Razors are really the only thing available and are probably great.
I think you would find more game with the 15s in either a Swaro or Meopta than you will the 18s in a Kaibab.