The 6.4 walks softly and carries a big stick. The 6.0 makes all the right noises but doesnt strike fear in the hearts of Duramax drivers like a tuned and deleted 6.4. But Im biased, and always error on the side of power.
Then again you have to ask yourself how much power do you really need?
As a matter of fact, of the last 104,000 miles all but about 200 miles the truck its been run in stock tune with more than enough power in stock form (unlike my 7.3 or 6BT Cummins). I only really turn it up if Im "showing" somebody what the truck is capable of or racing.
I believe if you have to ask, your mind is probably already made up, I know mine was once I rode in one.