I may be a little biased, but I would bet on the CAT motor any day over the Cummins. My bread and butter is the big truck market, and where I work, we actually hot forge all the crankshafts for the CAT motors that are assembled right here in Illinois. I have several friends who work at CAT as well and I can attest to the durability of their motors. (We also forge many Cummins crankshafts as well). As for the tranny, it's most likely an Allison auto. Both my Dad and Grandad (who was in charge of quality) both retired from Allison's in Indy and they really have the market on big truck auto tranny's - great track record. I grew up on Allison tranny's, and now I make my living off of CAT crankshafts, so pardon the bias, but if you've got a CAT/Allison combo, go for it! You won't be disappointed, even with the mileage.
Just my .02 worth...
Ken