I've owned several M939 series trucks (M936A2, M932A2, M929A1, M928A1, M932) and currently own a few more (two M925A2s and an M929) so I have experience with both motors. While I love the NHC250, I prefer the 6CTA 8.3L motor. The 6CTA just seems more snappy and it sure is much quieter. For those of you familiar with a Duramax, the motor is almost that quiet. Let's just say that no hearing protection is needed and you can talk on the cell phone at highway speeds. I would never turn down a clean M939/M939A1, but, all things being equal, I'd take an M939A2 over the others. With that said, I think you will have more luck finding one of the older trucks...and, as EMMADO22 said, the CTIS can be a nightmare. I have found that the NG frequently just plug the CTIS off, but leaves everything in place so it looks correct if inspections are conducted by the brass. One of my M925A2s is that way...