Sora is a good fighter and he is one of the chosen ones to hold the keyblade but Riku is the chosen one. Sora just became it later because of a cheesy scene where he had to win since he’s the main character. I think Riku definitely wins this match since he’s just like Sora but stronger. Of course in the game you wouldn’t know this since Sora always beats him. Riku wins.
But now Sora has Master Form among others. Riku won’t be able to win. Sora wins.
DReager1.com 
Except that Sora’s Heart is stronger than Riku’s. Therefore Sora beats Riku. Riku always wins in pure force simply because he’s cool like that, but Sora has the indomitable will.
Sora’s heart definitely was strong, but Riku’s has mastered Light and Dark. Most heart’s can’t take that kind of power. Of course Sora will put up a good fight