Depends if you need 6gb or 12 ggb vram. Titan Z is 6gb for Octane and Titan X is 12gb.
If you think 6 gb is enough, then it also depends on how many cards you can stick in your Build.
Titan Z has more renderpower, but depending on your config you can have more X or more Z.
With watercooling i.e. I can have 3 Z's in a current config instead of 2 with aircooling.
So, a config on water with 4xZ will perform better then 4xX, but has 'only' 6 gb.
And it depends on your budget, and the Z's are hard to get these days, so probably you end up with the great X
This is a nice thread about choice X or Zviewtopic.php?f=9&t=45951&p=231966&hilit=Titanz#p231966