Basically the UD3 has a 1394a port otherwise they are about the same. The U3 does not have a 1394a port and is an Ultra Durable 2 design and the UD3 is an Ultra Durable 3 design, though it's hard to tell much difference between design 2/3 other than the color of the mobo.
I wonder if you did buy the D3 one afterall? If you did can you tell me the feedback cuz I'm sorta facing the same dilemma now. Is it worth buying?
Also, what processor did you use on it?
phenom II x4 955 no oc
i bought it
i would recommend it to u if u will buy cooling system
cooling is not what i expected on this motherboard
i had ASUS M4A785TD-V EVO before i go to gigabyte and its much better in cooling
and i wish to go back to asus agian
M5A99X EVO is good deal for u