I'm collecting parts for a new build, and have narrowed my motherboard choices to the ASUS P8Z77-V Pro and the Gigabyte GA-Z77X-UD5H. I'm interested in feedback regarding how they compare in terms of quality, reliability, and stability.

Here's some reading to start with: http://www.bit-tech.net/hardware/motherboards/2012/05/09/gigabyte-ga-z77x-ud5h-review/8 (comparison is the Gene, but that is still a Z77 board from Asus). I'm sure you've read the Pro's review on SPCR already.

I haven't heard any bad things about either board yet. Personally have had less setbacks with Gigabyte boards than Asus, but that's just my individual case, and I've liked my last two Asus boards very much. The Asus has verified fan controlling functionality, but the Gigabyte EasyTune might do a decent job as well. EasyTune fan control is not explained in the board's manual, so you'll have to look that up somewhere.

Both boards claim lots of things and have multi-phase power designs, only time will tell which fares better. My pick of the litter was the mATX P8Z77-M Pro.

Personally I'd choose the ASUS because of the FanXpert 2. Have you read the review of the ASUS board with FanXpert 2 here at SPCR?

But if you don't intend to use the motherboard fan controller then I'd assume both boards are equally good. Then it boils down to personal preferences I guess.

I did read the review here. Do you know if it will work on the ASUS board usig 3 pin fans rather than 4 pin fans?

It's also useful to read the discussion thread and any product threads there are. This question is a frequent one. AFAIK the CPU header(s) are PWM only and the rest do both voltage and PWM.

PS. The article discussion thread even mentions EasyTune: "Just to chime in here... I just built an Ivy system using a Gigabyte Z77X-UD3H mobo. Fan controls in the bios and the windows app (easy tune) is extremely rudimentary. I use speedfan to control the fans. Also speedfan has no problem controlling 3pin fans plugged into the fanheaders."