Motherboard manufacturs can choose different companies to provide the various on-board peripheral chipsets that power things like SATA and USB ports. There are a couple different types of SATA controllers, each of which are distinguishable using the color-coded SATA ports (see above pictures). The four black ports are connected to the Intel PCH 3 GB/s controller, and the two whites are the 6 GB/s PCH component. The ASUS P8Z77-V Pro board actually has an additional ASMedia 6GB/s controller.
The Gigabyte Z77X-UD3H does quite well in SATA 3GB/s throughput, and is essentially identical to the P8Z77-V Pro on the Intel 6GB/s controller. If you plan on using more than two SSDs, however, you might be looking to jump up a price segment, because you don't see an add-on SATA 6GB/s controller as we saw on the X79-UD3, which offers performance almost exactly between 3 and 6GB/s throughput.
As for USB peripherals, the Gigabyte board is closely matched with ASUS in USB 2.0 and beats out the P8Z77-V Pro in both read and write in USB 3.0 by almost 20 MB/s write and a staggering 75 MB/s in sequential file reads. However, Gigabyte hasn't included any USB 3.0 "boost" capabilities which we saw on their X79-UD3 board, and it loses out on the potential performance gains of varying USB 3.0 protocols.