For me, it'd have to be Riku, just because i loved the command styles in BBS. But in terms of battle, i'd go with Sora, since in Critical mode, well timed links can save you due to Invinicibility frames, and you can pound the annoying bosses to death with them(Meow Jesty's Dual Link attack destroys the bosses).

Sora's, they seem to have more diversity and unique attacks, I felt with Riku you just powered him up slightly for a small period of time.  

I loved this the most.

 

I guess Riku's were cooler to look at, but Sora's were far better. Using the bouncing cat alone can absolutely ruin a boss's HP on Proud mode.The Dream Eaters are totally broken haha

Sora. He got to do a ton of stuff with his spirits and Riku's were more bland.

I hardly tried them. lol

SORA

But I enjoyed Ghost Drive

I loved Riku's. It was so slashy slashy.

I liked Riku's more but I found myself hardly using them on the final bosses just because of the fact that it almost seemed useless, and it gets rid of your dream eaters, so you could potentially miss a Spirit Roar or a combo assist, in which case you wouldn't do half the damage you'd do when not linked.

 

Sora's on the other hand... Well in doubt, use a meow wow to bounce Xemnas to death.

I felt like Sora's were better for me because he was kinda invincable. Plus nothing beats hopping on a meow wows back