Should I get Chongyun who I don't have or upgrade Ningguang's constellation with the free character thing?
I'd suggest looking on you tube for Chongyun team comps, because he's very good with some characters, but actually lowers the damage of other characters. Razor is famous for being horrible with him, because he turns the damage of your main DPS into cryo. If you don't have and like the characters he works best with, you might not want to bother.
Ningguang's C1 and C2 are supposed to be really good, though, so that's a tough decision. It's hard to pass up a new character for a constellation, but if you don't see yourself using that character and the constellation will be getting a lot of use, it might be worth it.
I'm in a similar situation with the same event, so I know where you're coming from here. For me, I'm trying to decide on Xinyan or C1 Ningguang. On one hand, I might end up pulling Hu Tao, and she might end up working well with Xinyan. On the other hand, it's pretty doubtful she'd be better then pairing Hu Tao with Bennett.