I'm with Doc and BCCC, the 7.3L and 6.0L Powerstrokes overlapped in the '03 model year (just like 5.9's and 6.7's overlapped in the '07 Dodges). The 6.0's had their issues, especially in the '03 and '04 model years, but the '05 - '07's were alot better. The vast majority of the problems were with motors that were heavily modified, or abused, or both. My '05 has over 80K miles with no major problems.
I did get the chance to drive an '08 F450 crew cab 4x4 King Ranch with the 6.4 just last night. It had the computer mileage display, and I never saw it show over 13.2 mpg, and that's cruising empty at 60 - 65 mph. Realizing though that it is an F450 with 4.88 gears, 19.5" wheels and a #14,500 gvwr - an F350 should be better, an F250 better still... It is however, far and away the nicest one I've ever been in - gps/nav system, reverse camera, not to mention the common stuff like adjustable pedals, power sliding rear window, moonroof, dual climate control, you name it, it has it. It would be a great ride to take to shows...
Mileage seems to vary as much from one individual truck to the next as it does between brands. I believe driving style has as much to do with it as anything. They all build good trucks, but they've all got their issues, too - especially the first year or two of a new engine or drivetrain or body style or whatever. Buy the one you like best, that'll give you the best odds of being happy with it.