If you are playing competitively, they both have different niches in today's metagame. Sigilyph is a troll with Cosmic Power, Wonder Guard, and Psycho Shift, while Xatu can bounce hazards back. It really depends on what you need and what fits your team better.
If you are playing in-game, you will obviously want Sigilyph as Xatu's special attack and speed are less. Plus, Sigilyph has better defenses and HP, and I personally like it's movepool better.
Hope I helped! :)