Hu Tao would be my choice, but there are many factors that could influence this decision (more details later).
This is because Pyro reactions are more powerful than Anemo reactions. You might be wondering why Klee or Diluc have the highest levels of pure DMG in the game. Both are 5-star Pyro users (ok, maybe a Pyro Catalyst/Pyro Claymore couple is also OP). Hu Tao also joins them, except she has a Polearm. This doesn’t apply elemental status as enemies like a Pyro Catalyst or Claymore, but it’s fast. (Xiangling has the same combo, but she’s a subDPS so she’s not relevant right now). Read more.
Back to my original point. Xiao uses a polearm, but his only reaction is Swirl. I feel that Hu Tao’s Pyro outweighs that. Pyro reacts to Electro, Melt (with Cryo), Hydro (with Hydro) and Burning (with Dendro). Pyro is the only element that reacts to Dendro so it’s easier to break Mitachurl shields or burn down thorns with Pyro.
These are some points to be aware of:
Are you equipped with the right weapons to handle either one? Hu Tao shines when she is given a weapon with a CRIT rate (i.e Deathmatch) or, even better: the weapon that was actually designed for her (i.e. Staff of Homa), to counter Paramita Papalio's HP eating up her HP. You will get the best out of Xiao if your DEF-boosting weapon counters Bane of All Evil's HP (i.e Deathmatch) or the weapon that was specifically designed for him (i.e Primordial jade Winged-Spear). Although you can use other weapons, it is much easier to choose to have the Staff of Homa and the Primordial Jade Winged-Spear. Both are extremely difficult to obtain. What type of element do you lack? This could be an important factor in your decision. If Traveler is your only anemo character, but you have many Pyro characters you may want to consider Xiao. If Amber is your only Pyro character, but you also have Jean, Venti and Sucrose, I would get Hu Tao. What are the other members in your team? Xiao would be most comfortable working with a team like the following: Sucrose to charge Anemo batteries, Zhongli to shred and Barbara to heal (or Jean, who could double as Anemo healer). These characters will make it easier to build a team with Xiao. You could make Hu Tao a team with the following characters: Zhongli/Diona, Xiangling (subDPS + Elemental resonance), Xingqiu (+small heals + constant Vaporize), Sucrose(shred), etc. Bennett is a good healer, but Hu Tao's attack bonus will be deactivated if Bennett has more than 50% HP. Whose design, playstyle, lore, idles, etc. do you like better? This is a key factor when choosing a character, especially a main DPS. It doesn't matter if you don't like playing with them.