...................Percentage of
............... First Class
..................Seats On Aircraft

Northwest Domestic Fleet
A319 - 12.90%
A320 - 10.81%
757-200 - 12.09%
757-200 - 11.96%
757-300 - 10.71%
DC-9-30 - 16.00%
DC-9-40 - 14.55%
DC-9-50 - 12.80%

Delta Domestic Fleet
737-300 - 9.68%
737-800 - 10.67%
757-200 - 12.90%
757-200 - 14.13%
757-200 - 12.22%
767-300 - 9.16%
MD-88 - 9.86%
MD-90 - 8.00%

If you fly on a Northwest aircraft, you will have a much better chance for an elite upgrade because Northwest has a percentage of first class seats. For example, 8% of Delta's MD-90's seats are first class, while 16% of Northwest's DC-9-30's seats are first class. Unfortunately this generous upgrade plane is headed to extinction.

I avoid Delta's 767-300s which have a 9.86%, and always try for the 757-200 which can go as high as 14.13%.