Well, I don't thin Navistar felt they could compete in high horsepower. They do make the DT466 which is a great motor, though too small for road tractors.
The V motor is lower profile and shorter. The disadvantages are it's got a lot more parts. The rest of the engine parameters aren't really relevant to arrangement of the cylinders except that keeping the deck height down is needed to keep the width compact. The inline motor makes a lot more sense in a vehicle that doesn't have space limitations.
I don't know why they ran both the 7.3 and the DT360 in school buses, but I avoided the 7.3 when I was looking for a bus. I hate working on V motors. You have to get under it for too many things. It has twice the parts. It has an unreliable exhaust junction and a maze of piping. You have to pull off the intake manifold and turbo and drain the coolant and who knows what else to get to the injection pump. On the inline motor everything is bolted to the side somewhere. I'm sure I wouldn't notice the difference driving them, it's just the maintence that interested me.
Then again, I don't know how the Cummins in the pickup looks but it seems likely the engine compartment is full so depending on that I might go either way on pickups.