After I bought my bus, I was chatting with the ISD mechanics a bit and asked why they only had IH and Cummins power around the barn. The response was interesting: "I can fix two DT466s for the price of one Cat, and do it less often".
I've wrenched (very lightly) on Cats, but I never had to fix anything major, hence I cant speak personally to the $$$ issue, but that whole conversation left me happy that I went the way I did.
"You can finally say you have enough horsepower when you leave two black streaks from corner to corner"
(Mark Donohue, famed TransAm driver)