Personally I would give the 6.4 a miss. It may be better than the 6.0 for reliability but Ford does not have pending litigation against International Harvester (who made the 6.4) for nothing.
The 6.7 diesel seems to be doing OK EXCEPT for the very few reports of Ford not covering High Pressure Fuel Pump issues.
GM uses the same pump and has seen similar very low failure rates, but IS reported to be honoring the warranties.
If you're looking for a 2008 1-ton, I strongly suggest you reconsider a Duramax. Very few issues with that vintage.