At this point I'm being optomistic.
The 6.0's are problematic by design, that is a fact. Don't get me wrong, I love mine, and would keep it forever if it'd last like our 7.3's - we have a couple of them with over 300,000 miles without any turbos/headgaskets/egr problems. I thought I had one of the "good" ones until the check engine light went on at 49K miles and my turbo went south. I'm not sure if thats a Ford problem, Navastar problem, or Garrett, but when the dealer tells me they keep extra turbo's in stock because they replace so many of them, that scares me.
That being said, I think out of the gate the 6.4 is proving to be a better designed engine and they seem to have worked out a lot of kinks the 6.0 had - hopefully.
Knowing what I know now about the 6.0, I'd take the chance on a new design and get the 6.4.
I know that I'm going to get a new 08 within the next few months.