The Z-68 would be better to get because it would give you more options and while it may cost more then the P67 you would be leaving yourself limited with options on the P67 board.
The Z-68 allows you to have onboard graphics with the Sandy Bridge cpu plus there are more Pci-e lanes for added mulitple video card support and it a;so alows for overclocking of the cpu.
Some of these options you may use and some you may not but they would be there for you to make that choice and with the P67 that choice is limited or taken away.
So if your building a new system then I would go with th Z-68.