For a long time I have used ASUS boards in my builds, regarding them as the most stable and solid products on the market. However, for several recent builds I decided to go for AsRock boards, because they quality + feature to price ratio was simply the best. I am very pleased with their products, from Z77 series I have built on ITX, Pro4, Extreme 4 and 6. There are no issues to date and I can say that now I consider AsRock products as good as Asus. These are the quality products, not a cheap, featureless entry-level products made by Asrock 5 years ago.
Boards you mentioned have virtually the same features, there is a difference in CPU voltage regulator. Asus has 12 phases for the CPU vcore, 1 for memory controller and 1 VTT voltage. Asrock uses 8+2+2, but has a better audio codec than Asus. For overclocking on air, they should be equal.
My choice would be Asrock.
If you don't want SLI, Extreme4 has all the features of Extreme6, except additional PCI-E and chip to control more pci-e lanes.
and once again, I am uncertain lol, I really like the Ex6 look over the ASUS, and I heard the Ex6 was better OC (that might not even be true ) but I'm just nervous to go with them, since, well, it's AsRock .. I heard their customer services was appalling (I've never had to contact any manufacturer, so I don't have any personal experiences I could relate too)
Looks don't matter mate Performance should always come first! The mobo will be in the cabinet anyway!
As for OC, the Asus is a beast. That board is MIND-Blowing. I never really liked the Extreme6. Okay it's good - Nice features and all, but afaik it's not better than the Asus P8Z88-V or the Gigabyte Z77X-UD5H..
The Asus has an AMAZING UEFI Bios and comes with amazing softwares for real-time hardware handling and management. I highly recommend you get the asus. The board has great reviews too!
ASRock is an amazing company too btw. Just the Extreme6 isn't THAT good. ASRock is good for X79 boards and the Extreme4 - Probably the most commonly used Z77 board.
So, make a choice between the V-PRO or the Gigabyte Z77X-UD5H.
Ps: Go for the Asus Trust me, you won't be disappointed!