I second Drew's comments about Detroits... they are all garbage. The EGR DDC60 might be the worst medium duty diesel of all time. Horrific fuel usage, no power, poor runability. They first came out in 2005 and now they have finally figured out how to get the variable turbo and egr to work "ok" 7 years later at the end of their life span. I don't know much about the QSX's but the medium duty ISX is a beast of an engine that Cummins developed. Incredible durability and efficiency.... The 2000-2004 might be the best medium duty engine that was available at the time, although it was unpopular due to cost. The 2005's with EGR had some issues, and the DPF's are a nightmare... but the 2010's with the SCR system appear to be doing better actually.

The problem with all of these is they are throwaways after 7-10 years... Its quite a change going on where you can see a 55+ year old SD9 moving cuts of cars around and be reasonable confident it will be around longer than the brand new genset sitting on the other track.