I think I’ll go with Ultra Moon.
1) When B2W2 came out, I decided to pick White 2 despite having played Black initially. This allowed to experiment both in terms of exclusive content, as well as see the story unfold through slightly different events. I want to do the same here with Ultra Moon, which I consider more like B2W2 than Emerald or Platinum. Plus, I am trying to establish a headcanon in which the ultra versions are sequels to the first versions, so having a different legendary this time around makes more sense from that perspective.
2) So far I seem to prefer the UBs in Moon/Ultra Moon than Sun/Ultra Sun. And since UBs seem to have a bigger role in the ultra versions, I’m picking Moon this time. Also, I pefer Necrozma’s Wings of Dawn form rather than the other.
Really, the only thing that makes me really hesitate is the prevalence Ultra Reckon Squad members. Nerdy McNerdface said it all: Zossie appears to have the most interesting perspectives and it’s a shame she’s more focused on in US. Nonetheless, she should appear in UM as well, which is why I am sticking with it for now.