The biggest variation is the BIOS. BIOS options, usually voltage settings but also cpuNB multiplier options, are critical for a good OC. BIOS implementation seems to vary wildly between manufacturers.
Power regulation also varies a lot from manufacturer to manufacturer and is the main reason I haven't been recommending any but the most expensive (790FX) ASUS and DFI boards for AM2+. Gigabyte seems to have all the bases covered at a reasonable price using the 790X chipset.
Is the 790X-UD4P the best at any price? NO. All 790FX boards seem to have slightly better power regulation and, therefore, OC slightly better then their 790X cousins (from the same manufacturer, if available). So the Gigabyte 790FX-UD5P is slightly better but, IMO, unless you need the added features (more than 2x16 PCIe and two Ethernet ports) it's not worth the added price. The ASUS and DFI 790FX offerings are also better then the 790X Gigabyte but, IMO, again not worth the added cost unless you're willing to invest in better RAM, cooling, and PSU as well ...