Neither or both.
Most P55 boards are roughly equivalent in performance. The major distinctions are connectivity, board layout and pricing.
Why look at a Gigabyte P55-UD6 when its replacement (with USB 3.0 and SATA 6Gb), the
P55A-UD6 is cheaper?
Overclocking is more a product of knowing how to manipulate the boards BIOS settings successfully. It is also very much dependant upon the CPU you use. Some will overclock well, some others not so much.This is mostly down to luck of the draw- unless you know which production batches have a higher oc potential or use a lower core voltage.
I personally would recommend a board that recovers well from a failed oc attempt, has a reputation for stability, a good warranty and replacement service by the manufacturer, is well laid out and has the aforementioned USB 3.0 and SATA 6Gb connectivity.
With those parameters in mind the P55A-UD6 is a reasonable candidate. The Phoenix Award BIOS offers enough tweaking options without the overkill (for most people) myriad of fine tuning details found in some other board's BIOS's (EVGA Classified/FTW, Asus Maximus etc.).
Other boards worth considering are the
Asus P7P55D-E Pro, Gigabyte P55A-UD4P or Asus P7P55D/P7P55D-E Deluxe.
If this board is part of a new build, then you also may have the option of delaying your purchase for a couple of months, at which time the LGA1155 (Sandy Bridge) CPU/socket and P67 boards should make their appearance. A
50% overclock (from 3.4GHz stock to 5GHz) on air cooling using nothing more than simply raising the multiplier from 34 to 50* is quite impressive.
* The 2600K CPU has the ability to raise it's multiplier to 65 by all accounts -so 6.5GHz (100% overclock) although the cooling needed would be something more sophisticated than air.