I would go with diesel no matter what. I know with the Ford Powerstrokes (I'm sure you can get one for the Cummins too) you can get the programmer chips. That allows you to tweak your engine for either better fuel economy or more torque. Personally, I'm in the market for a good Excursion with the 7.3 diesel but they're not plentiful.
I think Marty (5th) is talking about this site.
www.warrentonselect.com.
They do have a nice selection!
Keith