Personally, I like the idea of Imperials fighting Dominates more than either of those two taking on Orks or Dark Eldar. Orks/Orcs are a sort of cop-out in my opinion. War is a horrible, brutal thing, and making one side essentially a bunch of savage monsters that want to do nothing but "Kill da 'umies" takes away from the roleplaying. 

Why would a character feel remorse killing an ork? Everyone agrees the galaxy would be a better place without them. They are literally not even human.
What about a recidivist? Sure, a lot of Imperials feel the same way about "traitors" as they do orks, but it's a little harder when there's a human face staring back at you, fighting for the same reason as you, with a home, family, a life. It forces characters to have a little more character. 

It also opens the door for interaction between enemies other than with bullets and las-bolts.

That's my two-cents. If y'all just want to have a good 'ol time slaughtering greenies or space elves, that's fine by me. I'm just vomitting my opinion.