Hi newcomer and welcome to the Tom's hardware forum.
The ASUS is a very good mobo, but the gigabytes support USB 3.0 and SATA 6Gb/s at a less price is not a good option, I would get the UD3R, BUT bear in mind that the gigabyte mobo don'r support USB 3.0 when the X-Fire is enable.
I'm not exactly picking the motherboard based on USB 3.0 support or Sata 6gb/s as I don't think I will need them and/or have a use for them, or at least for now.
I didn't know about the crossfire and USB 3.0 conflict though, now I will know.
so far I'm leaning towards the UD3R, because of the price.
Problem with the Gigabyte is that it puts a whack on your PCI-E lanes somewhat crippling your GFX options if you wanna maintain full USB thruput. What Gigabyte does is essentially sacrifice half of its peak bandwidth to enable 5.0 Gb transfers to the USB 3.0 and SATA 6.0 Gb/s controllers, while those with two cards must live with 2.5 Gb/s bandwidth limits on USB 3.0 and SATA 6.0 Gb/s controllers.
So if you have multiple GFX cards....
w/ Asus you get 5.0 GB/s transfers on USB 3 and SATA III
w/ Gigabyte you get 2.5 GB/s transfers on USB 3 and SATA III
Of course you need something that can pump that for it to matter.