I will honestly say that my 6.0 Vortec is the best gas engine I have used, but with that said if you can find a 5.9 Cummins I would go that route in a heart beat.
If your using it as a daily driver (as I am) I average 13.7mpg pretty consistently driving on the highway too and from work which is about a 15 mile one way trip. My brother in law has a 2006 3500 Megacab HO Cummins and gets 17ish hand calculated on the same trip. Beyond that for towing specs if you can find the HO version its 325hp and 610tq @ 1600rpm, the 6.0 gas makes it's power @ about 4,000RPM for max torque.
If you can find a good deal on a 6.0 I wouldn't pass it up. These are iron block truck engines, which routinely out last the trucks components with many guys pushing 200,000 miles without any major engine repairs.
Test drive a bunch, your driving in the Triton 5.4 now, the 6.0 wont be much difference in fuel economy. Find one that is comfortable with the most "must haves" and go with that one.
