If you are basing your assessment of AJ over GMac in the games that we have seen played out, then you are comparing apples to oranges. Yes AJ appears to have the stronger arm and appears to throw the long ball better. Having said that I also remember in the 2008 thrashing of Auburn that GMac came in for a JP Wilson and threw a 30 some odd yard TD pass to Maze and looked magnificent doing it.
I said all that to say this. When you do get to see AJ in a game it is against a whooped opponent, for the most part. UT is thin at every position on the field. To say that he is going to be the next Carson Palmer, Matt Leinert, Philip Rivers, etc... is a bit premature. While I feel very comfortable that AJ or Sims will be a good QB for us in the future, I am not ready to send GMac off to pasture and say that our back up is better than the starter. If that were the case, he would certainly be the starter.
It is the age old debate that has centered around UA football for as long as I can remember. I remember everyone being excited when Tyler Watts got to Alabama, or Brodie Croyle, or the fact that GMac showed more poise than JP Wilson, and now AJ has more "touch" and a stronger arm than GMac. It's all relative. So long as they all win, nothing else much matters.