I don't want to think she's a guy for certain reasons, despite the fact that she's a replica of one [yeah, that sounds pretty odd]. Nor do I believe that she's a true girl, for other certain reasons. Nor a mixture of both, please: that's actually quite out there, and it's creepy. I wouldn't say "genderless". I see it more as sex. "Gender" or "gender identity" is what you see yourself as, or act like, while your sex, it's what you are biologically in today's science [more so with identity problems, transgenders, etc., etc.]. Xion has no sex. But she does identify herself as a female socially [I don't know whether it's the fact that Sora's memories of Kairi has enough influence so she, in an unbiased point of view, sees herself as the "female" version of Xion in her eyes. It's pretty likely] . Considering her basis and component are memories...she is Sora's memories, to be exact, she's kind of a missing chain and link to the rest of his memories [that have no real biology, I'm sure] that are preventing him from recovering, means that Xion really, doesn't have a sex.
I wouldn't want to be a guy, and have my memories be female >.>. That would be awkward, and I doubt that's the case. It's the reason why "she" died into crystals, and was nothing, but gems.
Pack with the fact that everyone sees something different, Ven, doll, etc., depending on their connection to Sora, all Xion really does is take forms with the situations. Kind of like Ditto, except since it's memories, it's more like an illusion that covers a blank doll, basically.
Nomura said that what Roxas sees, it's what the audience sees. So that's the majority of what we know Xion as, and why we refer to her as a "she" to make it simpler. He also said that Xigbar sees Ven, but acts around Xion as if she was female to not arouse suspicion, because Roxas acts like that around her in turn. He even said that Riku, the first time, saw Xion's "true nature". I don't know whether that meant Sora or not, but it was definitely different than the female appearance because Nomura separates them. Saïx saw the doll. Xemna saw both "Xion", whatever appearance that was, and Ven. Xion even admitted that she's Roxas and Sora [the character we speak of here, came to terms with it], so that pretty much seals the deal as well. All three of them are each other. A body and soul, the original, and the memories. Xion takes a form of Kairi, but she's definitely not a Kairi like Naminé is.
Overall, I'd say Xion has no sex. Not a gender, but a sex.
At all.
I guess that in the poll would be "genderless"...?