I really like the name Serena and I've always been used to hearing that being her name, but after watching Sailor Moon Crystal I've gotten used to Usagi. I don't really have a preference but Usagi makes more sense I guess. But if I really had to choose I would pick Serena though.

I prefer Usagi better because that is her original name and that is the name that Naoko Takeuchi picked for her too.