I found the following moveset for Aegislash on Smogon:
-Sacred Sword/Iron Head
Is it better to take this with Sacred Sword (coverage) or Iron Head (STAB)?
I think (just my personal opinion) that it is better to go for the STAB. If you ever need the coverage you can simpely switch out. Also when you are in a battle where you don't need the coverage, iron head will do much more damage.
hope I helped you
Sacred sword all the time. You have shadow sneak for your stab anyway, but after a few swords dances the stab boost wont matter any more. Sacred sword is very important for extra type coverage, as you do not want to have to switch out and waste a turn, also losing your important stat boosts.