Hola rjemery -
xxx
Well, you go compare two airplanes that are quite different.
The NW DC9-30s (and maybe 50s) are 1960-70 era airplanes.
Their engines (JT8D-9 or higher dash number) are .58 SFC for fuel burn.
The A320 family airplanes are 1990 and up-to date technology.
The CFM-56 is an engine with about a .35 SFC efficiency.
Certain that newer DC9s (i.e. MD-87s) would look much better.
xxx
What to replace the old NW DC-9s with...?
Obviously, modern A320 or 737NG airplanes.
I am certain than both are very similar in efficiency/economy.
The bottom line is the financing end.
European carriers definitely favor Airbus products.
And US/Canada operators will probably favor Boeing products.
It is both politics, banks, and high finance.
And put in there the "merge" factor of DL with NW...
xxx
Happy contrails