1. For overclocking under 1.4V and under 4.2ghz, you shouldn't have issues with improper power delivery on the Foxconn socket.
2. Most Asus boards are still stuck with Foxconn socket.
3. The Pro model supports 8x/8x Crossfire and SLI vs. 8x/4x on the UD3R and it only supports Crossfire. So if you intend to run dual videocard, UD3R is not even an option. If you don't intend to do so, then compare UD3R to P7P55D instead ($149.99).
4. I'd go with the Asus board simply because it allows you to run full power saving technologies (i.e. Vcore voltages lower even at overclocked cpu speeds) on an overclocked processor.
"Despite everything, our CPU overclocking experiments on Gigabyte GA-P55-UD6 mainboard were a success.
Of course, the board?s inability to keep processor power-saving technologies up and running, when the CPU core voltage is increased, does tie our hands a little bit. Asus mainboard allows us to keep the voltage at its nominal level, increase or lower it, but if there is no operational load, it will lower not only the multiplier but also the CPU core voltage. On Gigabyte mainboard power-saving will work properly only in case the CPU Vcore remains nominal. However, the maximum overclocking for our Intel Core i7-860 processor we managed to achieve on Gigabyte mainboard was just as good as on Asus, which is a great result." -
Xbitlabs
If you are trying to stay on budget though, UD3R is top notch at
$130
Although for light overclocking even Gigabyte UD2 will do.