"I would be curious what makes some torque usable, and other torque not usable."
I will be woefully inadequate in fully responding (i.e. let the engineers have at it) but in our application, we want to get all our "stuff" going from a stop. Most any diesel will have much more torque at a lower RPM than a gas engine, thus usable. The V10 gas engine has plenty of torque, but higher in the RPM range. Mostly a function of the fuel. If you aren't in a hurry, the Ford V10 will be fine, albeit with less MPGs.
Of course with the diesels getting better MPGs, I'm sure that will get evened out at the next oil change - 7 qts (gas) as opposed to 15 qts (diesel) and a $25+ filter. Apples and oranges!