Personally, I say this argument is invalid
A lot of us like Namine, such as myself, and a lot of people like Xion. However, I find this hard to debate over. I've played all games thus far, and truthfully I like both characters, but I like Namine more. I can somewhat blame this on the fact that we all knew Namine WAY before Xion, and while Namine may not have shown as much of herself as Xion did, my argument stands the same. I feel that because Namine had been introduced first is what causes my 'faving' of her, whereas I just don't really care that much for Xion. I like her, but I like Namine more. In this sense, had Xion been introduced first, my opinion would most definitely be different.
It's hard for me also because this argument seems completely unfair. A comparison to this argument would be the TifaCloudAerith argument for Final Fantasy VII, but the difference being that both Aerith and Tifa had been introduced in the same game AND both had different, yet powerful moments with Cloud. In THIS particular argument, however, it's almost easy to say that because Xion is new, she'd be more likeable and popular. My point is that because Xion was introduced after Namine, Xion is considered the new 'trend' in the Roxas love debate.
However, some like Namine for the exact same reason, but see things differently. Namine may have been introduced first, but that gave most of us time to grow attached to her. Because of this, the creation of Xion only outrages those who have grown so attached to Namine prefer her to be with Roxas. Xion fans, however, like her because she's new and has shown more.
There are many things that can, have, and will be debated on between the 'compatability' between both characters and Roxas. However, I will not choose between because in my opnion, this argument is unfair. Both girls are complete opposites. Namine is shy, fragile, part of Kairi, and in actuality kinda useless in battle. Xion is confused, whiny, part of Sora, and more useful in battle than Namine.
You know, when I really think about it, this whole affair is kinda wrong. if namine were to be with roxas, that would mean she'd be with sora, roxas, AND xion! if xion were to be with roxas, she'd be with roxas, sora, and... herself...? this is all some weird twisted threesome madness with some possible masturbation going on here! XD all jokes aside, I personally think roxas should be with namine and xion should be with axel XD i dunno why. I just like it that way! <3 Please don't flame this post!