Roxas would win. First off everyone says Sora would win with his drives or some ability but does nobody understand that his drives come from some magic clothes witches gave him? Just give Roxas some of those clothes and he'd stomp Sora. Roxas has a ton of solo fighting experience as well. Roxus took out Riku like it was nothing then when he let his guard down Riku pulled some bs sleeping move. Sora would have had to go into a drive just to be on par with Riku.
When looking at fighting experience:
Roxas has a ton of solo fighing experience.
Sora has a ton of team fighting experience.
Keyblades:
Roxas has the two best keyblades at his disposal.
Sora has a keyblade.
Roxas would just win hands down. Using some excuse like "he is Sora's nobody" makes no sense. Being Soras nobody doesn't make him weaker. He is his own person that has gotten stronger through his own experiences. In the KHII minifight Roxas wasn't even putting effort into it. He could have ended Sora but chose not to; I mean he literally disarmed Sora and had a keyblade at his throat. Then Sora re-summons his keyblade to slice Roxas like he accomplished something.
Roxas > Sora