They both play for the Jazz and I suspect they will be for a while. Therefore, it's less a matter of which is better and more about how they can each play to compliment one another's game. I think they can do that and not have to compete for the spotlight or accolades.
Who cares? This is the absolute wrong question to be asking and the question that needs to be asked in the minds of management is 'who has a higher ceiling: Al/Paul or Favors/Kanter?' Neither will reach their ceiling until Al is moved. If we lose Paul and retain Al, you can bet big we're eyeballing someone like Elton Brand to 'fill the void'.
That's the easy answer.
Sometimes the easy answer is the correct one.
Favors.
I love Kanter, and always have (unlike some of you), but if I could magically make one of them reach their absolute potential by tomorrow, it would undoubtably be Favors.