Which motherboard?

the UD6 was higher up the price ladder originally, but the UD4 is newer as denoted by the 'A' after the chipset (p55), and as such has some newer features (USB 3.0, SATA3 (6gbps), possibly others). if you need USB3 or SATA3 get the ud4, otherwise I would expect the UD6 to have a better all round specification, especially for reaching higher overclocks
 
At the end of the day it comes down to what you will be doing with the board. As Trojan said the specs of the UD4 are newer SATA3 and all that. But the UD6 does offer some outstanding benefits. Take your time and check and see what features you will use and which you will never use. You might find that the higher spec board (and cheaper) has a much longer upgrade path if you are not going to use SATA3 hard drives any time soon. USB3 is not a deal breaker for me.

Me I would go for the UD6 every day of the week.
 
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