I'll throw my two cents in.  I traded my 2006 6.0 two weeks ago on a 2008 GMC 3500 DRW.  Over the last year I have spent well over $6000 "Bullet Proofing" the engine and still left me stranded twice with other issues.  Luckily I run a Scan Gauge II and caught the overheating EGR before any damage was done but it ruined a vacation.  
I had been looking for a dually for awhile and found the GMC in Georgia for a $4000 under book with impecibleo maintenance records.  When the dealer saw my F350 he said he had a customer for my truck and offered me $19000 for it.  Well if dealer is offering me that much for a 6.0 I figured it was meant to be.
Let me tell you, the difference in performance from the F350 to the 3500 is night and day.  I have yet to pull my fiver but bobtail its like driving a sports car compared to the F350.  The 3500 rides smoother, is 100% quieter, more low end torque, and is more comfortable than the King Ranch and the wife likes it better.
Yes, I liked my F350 but was nervous driving it, afraid of what would happen next.  We plan on camping further and this was not a feeling I needed each time we headed out.  
Good luck on your decision.  I know I made the right on for us. We plan on keeping this truck for a long time and feel more confident that it will.