I don't think this is a fair poll. Serena has gotten plenty of exposure and you can see more of her character as the anime is getting both subbed by fans and dubbed. Y is pretty much an unknown to many as the X/Y arc is only available through raws, as I believe no one has been willing to pick up Pokemon Special after anon scans retired from scanlating barring Golden Pidgeot who has translated a couple of chapters. Even then the raws are mostly phone shots. Also, I don't believe Viz Media has started the translation yet.
That said, as someone who has read the raws for X/Y. Y is an easy choice for me. She's confident, and isn't afraid to voice her thoughts. She reminds me a lot of Sapphire just toned down a bit. Plus she has a Sylveon! The moment when she was trying to do Pokemon-Amie with her Eevee was pretty funny as well, and adds more to the bias.
When I first saw Y , I shout WHY SERENA ISN'T LIKE HER ?
I should us why the Writers aren't like Hidenoir Kusaka ???
That One Guy alone came up with Unique character like Gold , Dia , Plantina and Y !
While Pokemon Anime has 5 or 6 writer working together yet They can't create One good character.
I wonder if Serena still has the chance to be like Y ! Confident , Determinate , Strict and strong willed.
She shown the potential to be like Y.
However The moment she met Ash, She become like one of those annoying Weak Shoujo heroin who has major one side feeling toward a Kind guy for his One act of Kindness toward her.
I hate those Shoujo heroin !