I think you should go with whichever ever one would make you more happy. Do you like the charged shots playstyle or the highly mobile/plunging playstyle.

Both will probably do fine on the damage side, although I suspect Ganyu can reach a higher damage ceiling because she can do reverse melt on pyro for 1.5 times the damage. Xiao is anemo, so only swirl is possible.

BUT that shouldn't be your decision maker or breaker since for all intents and purposes both will do lots of damage compared to the entirety of what enemies we face. The content isn't hard enough to justify picking anyone over the other out of ganyu and xiao.

It's a PVE gacha, not a competitive MOBA game where optimization is everything. Fun though is almost the most important thing.