Assuming you're referring to the ASRock Z75 Pro3 and the ASRock Z77 Pro4, I say they are both excellent boards for the money.
I think I stated this on one of your threads already. The Z75 received a promising review from tweaktown. It can easily allow your CPU to be overclocked in the 4.5GHz range.
The Z77 Pro4 would be a better buy. It has more power phases as well as more Sata 6Gb/s ports. This may come in handy if you want to add more Sata III devices such as SSDs or HDDs. The power phases will help with overclocking.