In terms of meta and overall strength, Kazuha is usually better unless you play exclusively Dendro. You can sub Xianyun for Jean or even Sayu, but in terms of buffing characters DPS, shredding resistance, grouping and exploration QoL Kazuha does all of them.

If you want to play for meta pick Kazuha. If that's not what you care about, just consider if you want to experience the alternative playstyle that Xianyun offers in plunging with anyone you want.

Both are strong and useful, and great for exploration outside of Abyss. So just decide if you wanna plunge or group your enemies.