User blog comment:YourPrivateNightmare/Discussing the new defensive mastery tree/@comment-973379-20150626081219/@comment-26044269-20150626085522

AA -> 4% bonus AM OR MR for 1 point = 4% bonus for 1 point EA -> 10 bonus AM AND MR for 4 points = 5% bonus for 1 point

Although their efficiencies aren't that different (considering they're masteries), I think it's more of the case of there's better ways you could spend that one point which goes into AA, or at least that's what I think YPN is trying to say.

I personally take AA.