That's really a tough one... Matt Cassel played great this season and proved that he was the leader this team hoped he would be. But I don't think his leadership was fully in question. His statistical performance improved dramatically and that's where it matters most.
Bowe was excellent. I've questioned Bowe a lot, but that's because I'm a big fan of his. He started every game this season, set some Chiefs receiving records and put a lot of doubters to shame.
My vote goes to Bowe.