I am assuming these are N/S. Either one could run specials, but that is always a gamble. I am a fan of booking now, especially since you need six tickets.
PROS & CONS-
DELTA tends to go up, and only up, after their initial listing, so if you pick Delta, book now!
AIR TRAN does have a habit of canceling unpopular flights, then sometimes adding them back.
For example, they canceled our N/S flight into DTW last Feb. and put us on a return flight that really stunk. Early out of MCO (9AM-ish), with a really long layover in ATL, getting into DTW around 6PM. I first went to Air Tran's website and found the flight I wanted from the available flights, then I called Disney (I booked it through Disney, so I had to call Disney) and they put me through to Air Tran. Air Tran re-booked us on my preferred flight, no charge.
One caveat: the flight I wanted was the least expensive option at the time I switched it, this made them happy to switch. That flight did go up within a short time, so, if you need to do this, strike while the iron is hot.
I ended up with a flight that departed MCO 4:30-ish, arriving DTW 8:00 pm-ish, with a small lay-over in ATL. Much better. More Disney!