IT has been a while since I last flew on MX longhaul, but in terms of service and professionalism, they have been OK for a number of years. The 767s they fly use "pod" seating with less than flat recline.
For AAdvantage flyers, they offer bonus miles iirc, whereas BA doesn't. For some FTers, that would make the difference - for others, the BA Club World flat seats would.
In coach, MX offers 2-3-2 seating, but all coach is 7734.