Depends on what you are looking for.
Maximus IV GENE Z should be able to overclock slightly higher and gives you the benefits of Z68 (like Virtu and the ability to use the iGPU). It is also a micro ATX board, so you can do a smaller overall build. The BIOS of the GENE Z also gives you more options, such as full control over the VRMs.
However, if you are planning to SLI or Crossfire, the P8P67 Pro is better because of the 3-slot spacing (for better cooling in multi GPU setups).
Some of the more intricate options in the GENE-Z that you may not find on the P8P67 Pro:Edited by 996gt2 - 4/16/12 at 8:42pm