H67 = iGPU support but no CPU overclocking
P67 = no iGPU but supports CPU overclocking
Z68 = iGPU support and supports CPU overclocking, plus as a bonus it supports SSD caching for a HDD, so you can use up to 64GB of a spare SSD to boost the speed of a HDD. I used that feature on an older 64GB SSD to boost a 1TB Samsung F3 and kept my Steam folder (which is several hundred GBs if I have every game installed) installed to that pair of drives while my OS and other apps were on a 128GB Crucial M4
I have since moved on to a pair of 256GB Samsung 830 SSDs in RAID-0, and thus really could get by with just P67 as I really don't care for any of the iGPU features
Z68 has a pretty small niche to where it can really make a difference, granted, its not like Z68 products are all that more expensive than P67, so it makes sense to just go Z68 unless you're simply not going to be overclocking, then it might just make sense to go H67.