Admittedly, I haven't seen any of the Steelers in the preseason, nor have I followed the battle in camp. But I know what Leftwich is at this point, and it isn't pretty. Certainly the safer pick simply because Dixon hasn't gotten much action, but I worry if Leftwich can really win you any games.
Now, as I recall, Dixon played okay against a really good Baltimore team last year. And I know he was great in college, which wasn't that long ago. This is a bit of a guess, but I don't think he can be any worse than Leftwich and might actually be better. Also, to steal the point from someone above, he plays a lot like Roethlisberger, so it might be a little nicer adjustment (don't know, maybe one of the players can illuminate on that more). So, I'd give the first start to Dixon, but keep him on a short leash in case I'm wrong and he does completely bomb.