I have heard alot of comments similar to this one by Mr. Norris but i feel compelled to respond.
I have a 2003 Excursion Limited 6.0 Turbo with 120k miles. This truck runs like the day I drove it off the showroom floor. I replaced the alternator at 85k miles but other than that and my oil changes, I have never had to have the vehicle serviced in any way for any issues at all. I love this vehicle.
In defense of Mr. Norris' comments though, I also owned a pickup with a 7.3 liter and it also never had any mechanical issues.
The 6.0 is much more powerful in both horsepower and torque so if you are hauling (Which I AM) then you may want to go with the 6.0. Also, the 6.0 gets better mileage due to it's smaller displacement. | 
