Favonius lance if she's supposed to be support and stay support, the Jade Spear might be better if you're thinking about using another spear character in a DPS role in the future and are gonna replace Xiangling.

That way the weapon you invested in would have long term value and not just be used on a support.

Of course the Favonius Lance is overall better in a support role, the decision for the jade spear may only be ressource based. With the jade spear you'd really have to run a energy recharge sands on Xiangling otherwise she'd be starved for energy.

The jade spear also makes it so you could build Xiangling in a hybrid way and not just purely support, i that's what you want.