I've seen loads of normal and 1 dark
What does fairyceus have over xerneas besides more balk and a bigger movepool? If you paired xern with something that let it set up easily, wouldn't it be a lot better?
In theory (I haven't done much battling), Xerneas is better than Fairy Arceus in that it can pose an immediate threat with Geomancy. Something with Follow Me or Rage Powder and that can take out Aegislash (Togekiss seems to fit the bill here) will allow Xerneas to have a much easier time setting up.