I am a Ford guy. They have always treated me right and so far I have had no big issues with any Ford I have ever owned. I also own a 2008 6.4 with around 150k miles that I am happy with overall aside from the fuel economy. There are far more good 6.0 and 6.4 than bad ones. But, they are less reliable overall compared to the competition for the same years. You are purchasing used presumably to save money and keep within a budget. So given that and you are set on a diesel I would weight my search more heavily to the Cummins or Duramax for these years. If you find a good deal on a 6.0 or 6.4 I would not automatically rule it out but from a pure cost of ownership and net reliability standpoint from a Ford guy that is what I would do.