I think you should go with Jeanne first. She'll be getting an interlude this May that will remove the self-stun on her NP. MHXA's interlude won't be until July and, even then, tnere is still a distinct lack of dedicated Quick support to bring out her full ability (though we do have Alexander and Atalanta for the moment).
Who to raise?
I was lucky enough to pull both Jeanne and MHXA from the banner with around 90+ quartz. I want to max out both of them eventually, but I'm kind of stumped as to who I should pour all those exp cards and ascension items into first. Any input would be appreciated!
What servants do you have right now` or what do you lack? If you need a great tank, go for Jeanne. If you need a damage dealer, go for MHXA.
Since farming exp cards isn't all that hard, for now just raise the one you need more for the event and when the event is over, farm exp cards and level up the other.
Also, check which ascension items you have. MHXA needs the newer materials which you might not have while it is easy as hell to ascend Jeanne.