Ok guys, nobody likes to have their beliefs shot down, but... I dedided to post this here for maximum exposure. Think of it as living up to what I tell my 6 year old son, if you are wrong, admit it and go on.
Anyway. I decided to do a little research. It seems a gas motor is capable of producing more engine braking due to the closure of the throttle plate.
However, a diesel is still pushing against its higher combustion chamber compression so should spin down faster, right? And I think that may be where my thinking got skewed. Any engine turning at a high RPM takes more umph to keep it spinning. In other words, cut the fuel and it spins down QUICK. And a diesel (especially non-turbo) will fall to idle pretty quickly. I remember the days of OTR Trucking and we would let the motor "spin up" on the down side of a hill to make it hold better (lower gear, more RPM) and if it still got away, hit the retarder.
I think what makes it hard to compare is the differences between most gas and diesel powered vehicles in terms of weight, transmission, final gearing and chassis. I know if I step off my 6.5 at the bottom of an off ramp I don't have to use much service brake, my mom's suburban you better grab some brake early. But a lot of that is transmission too, I bet.
So.....boils down to this: a gasser has to pull against the vacuum in the intake for the pistons to fall, a diesel doesn't....
are you listening, son?