I would agree that AM would probably be best.

I can speak from experience of DL's 763 F service from ATL to CUN. It's alright, but there is no meal and the flight is about 2.5 hrs. On AM you'll get a meal in F and probably a much better meal than DL offers on their transcons. Service, however, is always great on DL and they keep the drinks flowing all the way to CUN!