Keller has played, and played well against top-flight competition. He's a legitimate pro prospect, who has spent the last year absorbing the offense. He has a cannon arm, can make all of he throws, and has proven in practices that he can run this offense smoothly, in fact even more smoothly than the reigning Big 12 Player of the Year.
Freeman showed some potential this past year. He looks like he'll end up being a good QB. it remains to be seen however if he'll develop into the kind of talent than Keller is.
If I had to choose between three years of Freeman and one year of Keller, I would lean toward Freeman, his character issues notwithstanding. But for next year, I'd rather have Sam Keller running my offense.