It's a matter of choice in the end, though there are some issues with Yelan in vaporizing team because Xingqiu is still the king with Hu Tao. But, this doesn't mean that Yelan is bad at all. Some friends jokingly say that Yelan is a 5-star Xingqiu, but their kits are different. While their elemental bursts are a bit similar, their kits are unique on their own. In the case of Yelan, you get a damage boost to your character, and her abilities scale off with HP and provide an area-wide application. But, Xingqiu's abilities' are single target, healing, and reducing damage. You can get Yelan, but getting constellations for Xingqiu is more workable for free-to-play players and light spenders. So, feel free to use any of them, depending on the enemies you are facing. Still, if you have Xingqiu, you should give him a priority in vaporizing compositions. But combining both isn't bad; they will provide insane off-field damage.