Swirl from Virisdecent already gives plenty fire res reduction

So on an ideal case, I think Xiangling is outclassed by Bennet

Though Xiangling can proc Gooba way easier than Bennet can keep his ulti up

I won't comment on the fact you talked about Xiangling at 6 constelations, but ignored Bennet's, specially for Diluc, even though OP talked about having 6 constelations of Xiangling.

That's a shame. Sure the OP doesn't have him full constelation, but of them give 15% pyro damage, give Bennet enough ATK and his Voyage will give, easily, 1k+ atk, because his isn't "extra %ATK" like Xiangling's. It's raw ATK based on his own. With only a single constelation, he already gives so much of his own ATK it's not even funny anymore.

Anyway, at this point, Xiangling probably is better since she needs way less gear to be good, whilst Bennet needs high ATK paired with a bit of energy regen ( which by using Diluc, I think there won't be energy problems, because of both of them sharing fire element ).