mostly just the fact that the UD5 is not able to overclock as high as the UD7. If your looking to really push the envelope the UD7 will take you further, a better all around board. Plus the UD5 still needs another rev to come out before it stops sucking lol.
Main thing is the better vrm and vrm placement. If you dont plan on going quad crossfire you probably wont see alot of difference in the ud5 vs ud7. But really its a better quality board all around and there has not been nearly as many issues with it.
Basically its UD5 vs Asus Sabertooth and sabertooth wins
and its UD7 vs Asus Crosshair V Formula Z that are both about the same level of quality but I think the UD7 has the slight advantage due to rugged strength.
Really its 2 completely different levels of motherboard, if you can afford the ud7 you wont regret it im sure, if you already have a ud5 the only difference you will see is uefi and LLC which is not a great combination on the ud5 at this moment
. If they come out with a rev4 or some new bios updates maybe the ud5 will see its time again, but uefi killed the ud5.
If you cant really afford the jump to UD7 you would probably be happier with the sabertooth.