I just flew JFK-MEX on AM J and MEX-JFK on DL Y+.

AM 789 was nice with aisle-access lie-flat but the menu and food were comically bad. Service was decent as I can speak Spanish.
DL 757 in Y+ was also fine, with a 50% full economy cabin and I had the entire row and exit seats all to myself. I was impressed with the wifi, fast speed and only lost connection for 10min while over the Gulf of Mexico. AM wifi costs $39.99 for entire flight, which is a ridiculous ripoff.

I would pick AM for J seat in 788/789 for the lieflat, but otherwise DL for J/Y+/Y in narrowbody.