If you are at the stage of having only 2 6-stars and not a lot of 6-star rank up materials, I would not recommend ranking either of the two. You should still level them up to max in Rank 1 though, as they can certainly still help you.
Just wait. you'll eventually pull champs who are worthier to rank up. If you rank up either x23 or AOU, then when you do pull a better champ, you probably won't have the resources to rank up said champ.
Under most circumstances, rank up based on need. Don't just rank up b/c you want to or for the sake of "progression."
Aou vision needs awakening and high sig... x23 is amazing with prof x synergy and apocalypse horsemen synergy. I would go with x23 but i suggest you wiat for better options. If you dont have a regen champ take her up or wait.
Comments
If you are at the stage of having only 2 6-stars and not a lot of 6-star rank up materials, I would not recommend ranking either of the two. You should still level them up to max in Rank 1 though, as they can certainly still help you.
Under most circumstances, rank up based on need. Don't just rank up b/c you want to or for the sake of "progression."