I have the Maximus Extreme, but I love ROG boards, so I'm also sweet on the Gene-Z and plan to build another rig with one soon.
Things to consider.
1) Do you have an open / windowed case? Is the look important for your build? If so, Gene-Z wins, in my opinion. The Pro looks nice too, but I don't care for the light-blue color and much prefer the ROG red/black.
2) Do you intend to do maximum overclocking, or just mild / medium? The P8Z68-V PRO has more power phases, whereas the Gene-Z is mid-range as far as ATX boards go, though it is at the high end of mATX. The Gene-Z blows away other mATX boards especially the ASUS H61/H67 models, but wont quite outperform the full ATX PRO. It doesn't mean you won't get high clocks on the Gene-Z, but you just might do better on the PRO with its power phases.
3) Clearance of 1st PCI Express slot on Gene-Z _could_ be an issue when combining a GPU in 1st PCIX slot with a big CPU heatsink if clocking. If you DO ever do SLI, the top slot on the Gene-Z is close to the CPU, and gets tight. A pre-sealed water system like the Corsair hydro series would be better fit. The Pro & other full ATX boards have more clearance and it isn't an issue.
I'd choose the Gene-Z for typical setups, just because it is great looking and could be versatile in more cases, but for actual best performance and a overclock build, the V PRO wins in the end; they are both great boards and I hear of people getting 4.8+ on the Gene-Z, so a lot of it comes down to the CPU you happen to get. You should be happy with either with the right components.