That's a tough call. I do know that if you decide to go diesel, I would shell out the bucks for the 7 year 200,000 mile warranty from the get go.
I think I wouldn't mind having the 6.4 as long as I could find a way to monitor what was being pumped into the friggin fuel tank. I mean it's like playing Russian Roulette everytime you fill one of these things up.
I personally have never been a fan of the modular motors. We melt the manifolds clean off these things when you put some weight on them, down South.