whos better?

Auron. He's stronger in battle when it comes to physical attacks.

Auron's cool. He was a lot more useful in battles, because of his strength. His "story" was fun to see develop as well.

But Kimahri's all right, I guess. I used him for stealing before Rikku came in.

I prefer Auron both plotwise and battlewise. Auron was better for the reasons Tavrobel stated. Battlewise, I didn't like Kimahri having such a short SG section, you could customize him to things, but he lacked his own style, which was boring in my opinion.