Been running the 6.0 for years, both 2003 w/3:73 and then 2015 w/4:10 ... I like the 4:10 better for hills (or course) but it's got plenty of power, even in the Rockies.
We haul only about 7500 lbs (A truck camper and cargo trailer) but it gives us little trouble ... I'd stay with my 6.0L forever, even if we buy another truck.