Chris Daughtry will sell more records (Okayyyy...CD's...arrrgh!) than anyone else featured this season simply because he is the best singer and performer. In the end, however, I believe Taylor Hicks' appeal is limited in the same way that Ruben Studdard's turned out to be. It's been a longgggg time since white boy soul sold in a polarized market, and face it, folks -- just like Ruben, TAYLOR DOESN'T HAVE THE LOOK OR THE MOVES, and doesn't display the capacity to change.
My mom adores Taylor. Personally, I think he's just above karaoke quality, an upgrade of Peter Tom Lewis, the short, hurky-jerky kid from season three. And before you cite him as an example, remember that Clay Aiken underwent an image transformation in between audition and the final, and his singing style isn't dependent on showmanship.
OTOH, Katharine does have the look of a star, and at this point in the competition, she is a better singer than was Kelly Clarkson, who has grown into a legitimate Grammy-winning superstar. I voted for Katharine, the first contestant I have been moved to vote for since Julia DeMato. She's not perfect -- her upper register needs serious work - but she's got serious talent and serious potential, like Simon said. Kat will do just fine. She probably won't win, but she she shouldn't have anyway. Chris should have.
I could be wrong about Taylor if they can perform magic in the studio the way they did with Daniel Powter. Holy crap, he sucks (even more) live.
That said, I won't be surprised if KAT wins. She'll come out like a Vanna White spokesmodel in a low-cut dress and wave, and the Producers will crown her their new "Miss America", er, American Idol. And her singing deficiencies will be covered up by sound engineers, and they'll put out a decent record for the teens and young adults who will buy it in droves.
This competition will come down to whoever the producers believe will sell the most records if they clinch the title. The producers will arrange it that that person will win. That's my firm belief.