I myself personally like the new design better. I think that Serena's old hair looked too thorny and blocky, and her outfit was really bland. Her new hair suits her much better, and the current outfit is much more colorful and fits Serena more. (Though I am not too sure about the boots) I do not mean to go against anyone or offend.

I like them both alot, but the new one is awesome.

I love both of Serena's designs!

I prefer the new one.

Serena's new design showcased (pardon the pun) her maturity and growth as a Performer after losing her first Showcase. I like how it directly connoted a boost in her confidence level, and I found it refreshing overall. Of course that might have been because I was introduced to the XY series when she already had short hair, and thus to me her old design was actually the "new" design since I saw it later. I just love her new outfit, especially the ribbon, and I find it a welcoming breath of fresh air.