Personally, I'd get a later 6.0 ('05-'07) before I got a 6.4 due to the 6.4's known crappy fuel economy. Plus the fact that the 6.4 was only available for 3 years, so parts and stuff will be more expensive, down the road.
Or, if possible wait till there is used '11 trucks. I hear only good things so far with the new platform Super Duties.
The Hauler - '05 F-350 4x4 CC Dually 6.0 Powerstroke
The Project - '90 Ford Bronco 4x4
The wife's - '11 Ford Escape
For every minute you are away from work...you can add two minutes to your life!