I would go with Z68 because the features.
There are 3 major differences between the P67 and the Z68.
Intel SRT (Smart Response Technology) lets you to use a small SSD (<50GB) to work as a caching drive to speed up the boot, start up of apps and access times.
IGP Support (Integrated Graphics on Processor) the Z68 boards have build in support for the IGP. The main advantages of this is that it will allow you to use the IGP for things like backup graphics and help with trouble shooting issues. Also with the Lucid Logix Virtu software you switch between the IGP and the dedicated video card for the best performance for the application that you are running.
Intel SATA III controller is better performing then the old Marvell controller that is used on the P67 boards.
Z68 is also great overclocker!
Those are the differences between the Z68 and the P67.