At 37.5:
Rhydon 14/14/12: 3141 CP; 172HP
Rhydon 13/15/13: 3142 CP: 173 HP
So...it will jump the CP, but barely. Honestly everything is negligible. I'd just pump up the first one because he'd take slightly less work and if you like the moveset. He's not a great defender due to double weakness to water and grass, but can still claim a good spot.