Both can enslave xenos, but Authoritarians are allowed xeno citizens. Authoritarian ethics also unlocks Slaver Guilds, which will make every species partially enslaved (you can still fully enslave some xeno species, as long as you have enough citizen pops left for ruler jobs), and stratified living standards, which make it easy to maintain high stability even when most of the population is unhappy (as long as you have some ruler pops and keep them happy).
Xenophobic ethics have the advantages that you can spam outposts at a low influence cost and you get a bonus to natural population growth, but neither of those is all that useful as a Clone Army origin. Your clone empire should be conquering other empires, not trying to beat them in a colony race.
Edit: I think Resident pops can do ruler jobs, but as non-citizens they suffer a happiness penalty and don't benefit from amenities happiness, so you lose quite a bit of stability. So a xenophobic Clone Army could be workable, but I still think Authoritarian ethics are much more useful overall.