The benefits of Z68 are SSD caching, and a Lucid chip (on some Z68 mobos) which allows you to use the IGP, AKA, integrated graphics processor on the CPU which can be used to encode videos using Intels Quicksync and you can also use it to boost your graphics performance of your GPU or use just the IGP alone.
There is not much point that I personally see in Z68 if you are building a mid to high end system, if you have a decent SSD there is no need for the SSD caching and if you have a good GPU then there is no point in using the IGP. So I would say if you can get a P67 board for cheaper than what you can get a Z68 one go for P67, unless you think SSD caching or being able to use the IGP would be useful to you.