With the way things are at DL, you are right to pursue AA. They have a "challenge" where you fly X amount in Y time and you get either Gold (lowest elite) or Platinum (mid elite). I don't think there's a Executive Platinum (EXP, top elite) challenge. Search the American forum for the details, unless someone else is kind enough to provide them here.
Maybe one day I'll be able to say that Delta is at the top of frequent flier programs. Today is not that day. Sample the mood of the community at the Delta forum and you'll see what we mean.
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"Sire, it is not a revolt. It is a Revolution!"