I do know where Ballard is!

My "2nd family" while I was in high school now lives south of Nashville and it was my friend's dad that got me hooked on driving big trucks.
Heya Stealth!
Both Detroit and Cummins are solid motors from my experience with them. They each have their pluses and minues.
Detroit(as in GM) was languishing for quite awhile until they designed and solidified the Series 60 in the late 80's. Once they partnered with Penske(who I eventually worked for) it became the new "go to" engine for fleets that HAD to have the latest. The "padded" fleet business transactions helped even as buggy as the engine systems still were at the time. Still, huge profits allowed more experimentation and development, and the engine(and it's prodigy) became some pretty consistent horsepower for the lower RPM/fuel conserving crowd. The caveat to this was DD knew they could expand that profit margin and the cost of parts and repairs can reflect it. They also remained somewhat proprietary which made getting parts for breakdown repair a pain.
Cummins I think has been around since dirt was invented, or shortly thereafter. When I started driving the Big Cam Fours had become the rage for the ever cost conscious fleet crowd. Unfortunately, they became subject to overheating problems due to the "new" low flow cooling systems and crappy thermostats and it was during this momentary lapse in engineering genius that Detroit Diesel got the jump on them. It didn't help that the cooling system HAD to be in tune with the "Mechanical Variable Timing" or MVT and mechanics weren't particularly fond of it. Luckily there were still enough of the older Big Cams around that cooling system parts could be robbed, systems punched out and retrofitted and things could be made to work until Cummins redesigned things. I think there was a lifter or injector issue at this time as well.
They smoked, they leaked, they were cheap to fix! My knowledge of Cummins motors isn't as much as Detroits back then but I do remember the next generation of Cummins stealing back a good chunk of thunder that they had lost to DD.