Very close because I like the fact that AJ has at least some experience but I have to say it would be Sims based on what we know now, namely yesterday's performance.
The fact that Sims is a year behind AJ and the competition is this close, speaks volumes imho. I read somewhere that Sims worked hard in off-season to catch up watching film etc, and it appears thats paid off for him. I love that kind of attitude.
Imo, Sims reads the field better and makes quicker decisions in checking down as Alassipi mentioned and he doesn't hang on to the ball too long. That quickness in decision though sometimes catches the receiver not quite ready in the pattern, and causes him incompletions. He can zip it, and that was a problem for him early in the scrimmage yesterday with some drops.
Some have mentioned a low throwing motion although that didn't stand out to me yesterday. And the whole height thing is reaching imo.
AJ does have a tendancy to hold the ball too long only to scramble and try and throw it away or force an int. by trying to do too much; He still showed that tendancy yesterday and he was sacked a few times as well as some throw-aways for no gain. Statistically though, I think he did better and made some beautiful passes. I think he's more accurate than Sims on the deep balls as well.
The thing I love a bout AJ is though IS the "gun-slinger" mentality and not being afraid to take shots down the field, lord knows we've needed that in our offense at times, but its also that very thing that gets him into trouble.
As most have mentioned, there are differences, though none are glaring, but I think right now Sims has a better upside than AJ and seems to able to control the offense better in terms of moving the ball, reading the defense, and a little less tendancy to turn the ball over.
Hard decision and glad I don't have to make it.