The UA policy is better than the AA policy, because UA counts *redemption* as activity to reset your 36 month clock while AA does not count redemption.

Also, DL's policy is slightly more generous in terms of months in some cases because they only expire miles on one day (December 31), so whenever you fly, you have a minimum of 36 months until your miles expire (not 36 months).

BTW, does anyone know how UA and AA will be doing the 36 months? Will it be exactly 36 months to the day or the end of the month (so if you fly on June 2, your miles will expire at the end of June in 36 months), or will they go with the DL system of only expiring miles on December 31?