The Extreme4 and Extreme6 look to be very good boards. They offer quad SLI and many other features which i can't seem to find in any Asus or any other Motherboard brand for that price range.
There are only a few differences between the Extreme4 and Extreme6 which can be seen here: http://www.asrock.com/mb/compare.asp?SelectedModel=Z77%...
I have the ASUS P8Z77-V, really good mobo. I've had 4.3GHz on my 3570k at 1.172V with this board, I was planning to go higher but I then I though 4.0GHz was enough for me. I might go higher later on though. "Quad-GPU SLI and Quad-GPU/3-Way CrossFireX Support!" check the ASUS website. The ASUS only has 8 phases for the CPU (and another 4 the iGPU), but it does use Alloy Chokes, whereas the ASrock use ferrite chokes, correct me if I'm wrong. The TPU switch is nice if you're new to overclocking, but I got my CPU to 4.2GHz using that, with high voltages too. But yea, the ASUS P8Z77-V is a good mobo.
I prefer the ASUS P8Z77-V or ASUS P8Z77-V Pro for their extensive features, and quality of components. Example Intel LAN, additional digital phases to the DRAM, onboard fan control, etc.
NONE of the MOBO's listed offer 3/4-WAY SLI, and don't confuse 'Quad SLI' with 4-WAY it means they all support SLI dual GPU cards e.g. 2-WAY SLI GTX 690's.
All of the MOBO's will work, then ASUS with it's higher quality components will last longer on overage than most other competitors.
I don't take stake in one or two reviews with folks testing the MOBO's for <30 minutes, but instead look at the long-term, real user feedback, personal experience and a trend from multiple reviews from multiple reviewers.