At $84.99 the ASRock Z75 Pro comes in $10 cheaper than its big brother the Z77 Pro and for most users, they wouldn't be able to tell the difference between the two boards. So the question that needs to be answered is what's the better choice? I think from a pure common sense point of view, the Z75 Pro3 is thanks to it being $10 cheaper.
Saying that we'd probably almost lean towards the Z77 Pro3 as the board to buy simply because it's the "higher end" chipset when comparing the two and the fact that it's only $10 more expensive is almost worth it to just be on the said chipset. If you're extremely tight on your budget, though, you can get the Z75 Pro3 knowing that you're really going to have the same performance as a Z77 based motherboard.
The Z75 Pro3 ultimately shares the same flaws as the Z77 Pro3 with the biggest being the aesthetics. By no means are the Pro3 boards good looking, they are very practical, though, and hit at a very strong price point. The mean reason we're probably not seeing companies do much with the Z75 chipset, though, is because the difference between them is so tiny that in the end only $10 separates them and an entry level Z77 based motherboard.
The ASRock Z75 Pro3 is not only a good board, but compared to the Z77 Pro3, it's probably a better board due to the lower price tag. If you don't need Intel Smart Response Technology then this is really an easy choice, especially when you consider while one of the other limitations with the Z75 chipset is the PCIe x16 configuration which sees both the Z77 and Z75 Pro3 share the x16 / x4 setup.
In the end most people will probably find that the extra $10 isn't worth it for ISRT; what most people will probably feel, though, is that they'd rather pay the extra $10 to be on a Z77 based board verses a Z75 board.
Both boards are great for people on a budget, though, and in typical fashion ASRock really doesn't disappoint, be it with a board that comes in at under $100 or a board that comes in at over $300.