The preceding posts should have answered your specific questions on the comparative mechanics of AA's an DL's FF programs. My two cents is that in the mid-199os, fed up with scheduling, limited overseas routes and a few other shortcomings, I defected to DL at top elite level for 18 months. This was during the reign of "Clueless" Bob Coggins. Once I realized how bad it really can be at an airline -- some months the policy was hold flights for late connecting incoming flight, the next month it was just the opposite -- I realized, there was no place like home, no place like home...