I am also in the Ail camp for all the reasons Tsundereshipper mentioned and more. What's funny is that Usagi really is supposed to be focused solely on getting back with Mamoru, but when Mako starts chasing Seijuurou, Usagi gets super jealous and decides to follow
them.
Plus, the story potential is much stronger with Ail than Mamoru. First of all, there is an "enemies to lovers" trope at play. Even if redeemed and no longer needing to steal energy, Ail is still an alien, so there could be more of a navigation of Earth customs and other hurdles from that angle. As far as integrating a character with his powerset into the cast, one can easily nerf Ail by framing any depowering at a consequence of the tree becoming a sapling. Plus, with the idea of keeping the tree on Earth, there's always that danger that it could be corrupted again as a source of tension.
There's also the issue that since Ail is an alien and Usagi is essentially human if they could even be compatible long-term. Are their lifespans similar? Would they, if they decide to spend their lives together, have kids? (I'd argue the magic of the Silver Crystal would help with this regard.)
Plus, can you imagine if by the time the Daimons come around, Ail were to counter with a Cardian only for the Cardian to get wrecked? And then as Sailor Moon would start to get more powerful than Ail, would Ail be able to "keep up?" Unlike Mamoru who was content to simply become a virtual bystander in the later season finales, Ail is a lot more active. As the relationship would shift from him protecting her to her protecting him, he'd be forced to examine his role.
Finally, it would make more sense if Ail were to save the day with a rose, since he is essentially an alien plant person.