Well, Riku is about 10x faster than Sora. Sora would probably have to push Final Form to the max to be able to beat Riku, even then Riku is still able to use dark aura, and dark shield, despite him giving up darkness. PLUS Riku can use both light AND Darkness. Riku was supposed to be the original Keyblade master, but was too tempted by darkness. Even though the Keyblade forsake Riku, he still managed to obtain "Soul Eater" and eventually "Way To The Dawn" equivalent to Oblivion. Riku is just as strong as Sora, and a much better swordsman. Riku has more experience as well, not just being a year older, but also the year while Sora was in recovery. Riku was made much stronger by darkness, and even when he turned back to the light, he was much stronger than Sora, at the end of KH2 Riku still has the advantage.