I saw them both play twice last year when they were redshirt freshman...once at the spring game and once at their final scrimmage before the season started. Both played really poorly both times. I was unimpressed with either, but again, this was over a year ago, and QBs do improve (see: Golson, Everett, and hell Jimmy C as a freshman).
I personally like Kessler better, just because he can scramble more and seems to be the 'gamer' type. Think Tommy Rees with much more mobility. Wittek has the bigger arm and is much more of the prototype of USC QBs, and he's got some game and could be a real player in the future. But right now, not having Barks for that game is a real plus for ND, no doubt, despite his penchant for picks. He's still far ahead of the other QBs, and with our pass rush and ability to stop the run, having a brand new QB starting his first game is NOT good for USC...