I ordered my 2007 at end of 2006 so I could get the 5.9L as it was a proven reliable platform......never want to be 1st year guinea pig for new engine/controls.
We were going FT and I didn't need that worry or headache.....same reason I didn't go with a FORD at that time
The 6.7L went thru some 'learning curves' (software programing) and some turbo/variable vane exhaust braking issues. Replacements and cleaning ports installed along with operating procedure changes (leaving EB ON all time)
Were among issues I heard/read about.
In time the 6.7L has gained in popularity and reliability.
Still would go with my 5.9L :B