Both boards are very good overclockers. Personally? I would probably choose the Z97x Killer as it is most likely marketed towards gamers. In the end, it should really be a choice to match the color scheme of your build.
I have an
Nzxt phantom 530 white
With the galax 980 hof also white
But my coolermaster cpu coolers fan is blue and my case lights aswell so ill probably go with the exsteme 4