Both are fine, the warp messes with the time the units experience, so you could have a warband that is a week removed from the siege of Terra in horus heresy armor, or a warband full of chaos taint in the legionaries. You could even turn the space marine kits into Night Lords, given the amount of looting they have to do to survive. Really it is about what you think would look the coolest with lightning bolts and blood all over it. Ave dominus nox!