It's really hard to say which side of the story I prefer, but I did enjoy Riku's story and the message they're trying to display in his story. Learning to face your darkness and being able to channel it your way is certainly something you don't normally hear in most stories, at least to my knowledge. It's equivalent to saying learn to use your weakness as your strength.
Then, there is Riku as a character. Out of all the characters in the series, I feel he is the best developed character. However, this game is just scratching the surface. I will get to full detail when I eventually play another future game in my playthrough (so stay tuned for that). For now, his development in this game was the beginning.
As for gameplay, it is pretty much still card game play. However, he is somewhat limited. Sora has a lot of customization while Riku, on the other hand, is stuck with a stack of cards and there is no way to customize. However, the upside is that you can use a majority of enemy cards more often than Sora would (to which his cards have a cost).
Personally, I did feel more challenged and more satisfied with his gameplay. As mentioned before, the set of his cards is limited and you cannot customize your deck. Which makes the strategizing more unique compared to Sora (to which you can easily break the game with a certain combo LOL). BTW, I am not a fan of the final battle in his story....just throwing that out there.
With all that being said, I mentioned that I am more mid ground with both sides of the game. But if I were in a situation where the world depends on this particular decision in hopes Earth doesn't explode, I am leaned more to Riku's gameplay and story. His progression is more interesting and the gameplay is unique and challenging with the limited decks I'm given.
I'd give it 6.5/10