Ok, I'm gonna throw my 2 cents in here,
Having owned,driven,and worked both over long and short distances,hands down the Mack kicks the 250s posterior in every way but 1,top speed.
The 800 series trucks will manage 56 mph,the M39s with the Mack will top out at 52,I cannot explain why,same gear ratio,same trans,same governed RPM.
The Mack really shows it's power in hard pulls,usually up that LOOOOOOOONG hill whilst towing another truck(or loaded),probably due to it being turbocharged.
The 250 CAN be made into a better runner(turbo,button change,more fuel) but you can do the same with the Mack by turning 2 screws in the IP.
The Mack parts are more expensive,that's true, but run 30 WT oil with a high Zinc content and keep an eye on your valve lash and it will last forever.
As far as filters.belts,etc. The Mack has 2 oil and 2 fuel filters,plus 2 belts.
The Cummins has 1 oil,1 Fuel and 5 belts,so kind of a trade off there.
As to the in cab noise,the Mack wins hands down, none of that normally aspirated rattle.
The Macks have their followers,as do the 250s, but every time I had a choice which truck I drove,it was an M39 with a Mack!
