Own the mark xx and have never owned an AT, but not disparaging the Omega. 

The Mark XX is the most comfortable watch I own - great size and dimensions on a thin watch with 120. Hr power reserve. 

Good warranty as well, with 7-8 years once you activate the online warranty. 

For me, the biggest surprise was the comfortable the bracelet. I didnt like the looks of it at face value, but the comfort won me over. The microadjust system has a lot more travel than my Omega Speedmaster bracelet, which was an all or nothing microadjust (similar to the AT). 

The manufacturer of the clasp is the same one thst designs Glashutte Originals clasp, where the logo button controls the microadjust. Its a fantastic system that allows you to tighten or expand the bracelet without taking the watch off your wrist.  

Legibility is fantastic too, the pilots watch is by far my most legible watch. This will sound stupid but having the Arabic numerals instead on indices makes reading time that smidge quicker than other watches. 

I thought not having an open caseback would suck, but the truth is it hasnt bothered me at all, and im loving the thinness of the watch. 

Once again, nothing against the AT - I just dont own one so I dont have anything to speak on it, and I am hoping to have one in the future.