There's no real justification like "it'll pay for itself with better fuel economy!" Yes, it gets better MPG. Yes, you'll stop for fuel less often (which I find really nice). The big difference is the effortless towing, uphill, downhill, flat land. Just set the cruise control and go.
I went from a 2004 5.7 liter Expedition to my 2006 6.0 liter F350 diesel last year. Phenomenal difference with a 7,300 lbs. trailer.
I got the 6.0 because is was old enough for me to afford it and, although there were some problems with the engine, I personally knew people with five of these vehicles that haven't had a lick of trouble.