Since 2 different rate up banners of new characters don't exist at the same time, you will have to make a decision whether to summon for the current banner character, or wait for the next one. So which one is better?
Both Ganyu and Xiao are made to be damage dealers, just through different ways. If you aren't in need of a damage dealer, then you probably don't even need to roll for them, especially if you don't have enough resources. But if you often struggle with some content due to lack of damage, then it's recommended to try for 1 of the two.
Xiao's full information and numbers aren't officially released, but datamined information from honeyhunter gives a good picture for his abilities, but can still be changed for the official release. This page is mostly for discussion purposes and to help make a more informed decision. Obligatory message: "play the game the way you want, and chase waifus/husbandos if you like them, instead of chasing after the meta".
Charge Shots vs Plunge Attacks
Starting with the obvious, both their playstyles are quite different from each other. Ganyu's main way of dealing damage is through her charge shots, which only takes around 2 seconds to charge, and it's spammable since it has no cooldown. Xiao's plunge attacks are also potentially fast and spammable, though it's unconfirmed yet if he will work the same way as the old CBT videos. Xiao's normal and charge attacks are also quite fast, and more easily chained with his elemental skill, while Ganyu only has her charge shot. Her elemental skill and burst can help with additional damage, but not as fast/reliable as Xiao.
In terms of damage, if the datamined info is accurate/unchanged, Ganyu's charge shot deals more damage than 1 Xiao plunge, even with his elemental burst. However, Xiao's plunge attack spam is potentially faster than Ganyu, as well as being able to directly chain it with his elemental skill for an even faster DPS. Meanwhile, most of Ganyu's damage is focused on the charge attack.
Both can be fairly easy to use, especially since Ganyu's charge shots have an aoe to make it hard to miss, while Xiao is a standard melee character playstyle. The biggest difference between the two is their range. Ganyu can attack from a safe distance, while Xiao has to be melee. If you don't like, or aren't very good, with bow's aim mode to attack reliably using Ganyu, then Xiao may be the better choice for you.
Ganyu's Cryo element means that she has access to Cryo elemental reactions, including "Melt" for increased damage. Applying a strong melt on an enemy means that you can increase the damage of both parts of her charge attack by 50%, which is a pretty significant boost in damage, but requires proper setup.
On the other hand, Xiao can ONLY do "Swirl" reactions, which is not that good on its own. However, being an Anemo character means that he can use Viridescent Venerer artifact set for -40% elemental reduction to enemies, but only for elements that's not Anemo or Geo. This means that he won't be increasing his own damage, but his teammates' damage.