[theastronaut]

Either should "work", I think 1 1/8th is pretty much standard with a booster and disc brakes.

Smaller bore = easier pedal, larger bore = more effort needed.

If you don't have abs to do the thinking for you when you have to panic stop then its better to have a brake system that requires more effort so you can more easily modulate the brakes to keep them from locking up. If the pedal is light/easy you'll just jab the pedal and lock them up and slide uncontrollably in a panic stop situation. A higher effort pedal and some practice stops to get a feel for how the truck reacts under very hard braking could be the difference between stopping in time or sliding into something you don't want to hit.