IMHO, you would be crazy to stay with AA (for the next six months at least). Delta (or US) would be a better option.

That said, reading these boards, I wouldn't expect any upgrades as a DL Gold or as a Platinum even, going through ATL. You will get free bags and ... ?

Though I am biased, I would look at US. You can always credit the miles to UA, who has a decent FF program (and is part of the *A). US has 5 non-stops a day vs. 3 for Delta and all DL is on Mesaba or Comair. Mainline on US is 50/50. Plus, while DL has a better inflight product, at an hour and forty-five minute flight, I'm not sure you will notice a difference.