Z68 boards support integrated graphics and and have SSD caching (which is pretty useless truthfully) and are better for video rendering. P67 boards are generally cheaper than an equivalent Z68 board and I've heard several people on the forums say they have gotten better overclocking results using them instead of Z68s. While a Z68 board might have some nice features, the cheaper boards often have less upgrade slots than the P67 boards of the same price range. So if you go for the Z68 make sure it has enough PCI-E slots etc. for your build. Hope this helps
If it's cheaper than the P67 equivalent then go for it. Basically just P67 with added features anyway. Unless you need the features though, I wouldn't pay extra for Z68 over P67 though.
Sandy Bridge chipsets in short : H61 - only two memory modules, 4 SATA2 ports, no SATA3 unless extra SATA controller is added. H67 - up to 4 memory modules, 4 SATA2, 2 SATA3, IGP and QuickSync supported Q67 - H67 plus support for VT-d (device virtualization) P67 - H67 minus IGP and QuickSync, plus support for overclock Z68 - H67 plus support for overclock plus SSD caching So Z68 is the choice having most features, unless you need VT-d - in that case you need Q67 or C206 (Asus P8B WS) boards.