Even though M&S regulars hate the devaluation, I'm still hard pressed to think of a better option for *A regulars. Air Canada Aeroplan I guess. Less for status than for earn/burn.

*G status isn't like Oneworld Emerald or Sapphire. One soon becomes quite dissatisfied with the random lounge access and seat selection policies, no elite mileage bonus, etc. when trying to use one FFP card with a *A partner airline. If you are already flying J most of the time, I wouldn't be too concerned about *G - if you get it organically then great...