The center diff is just used for getting rid of the binding that occurs between front and back wheels, I'm sure you know that.
With a Part Time setup you have the wheels slip which has the same effect.

So when you're in a situation where the wheels can slip the two systems are pretty much the same.
However, just like in an Automatic Transmission the fluid coupling (center diff) on the AWD looses some of the energy transfered from the engine to the wheels. Since the drive train isn't solid from the engine to wheel you can't apply the torque needed in some situations.
Plus factor in having the front wheels go up a steep ledge or other obstacle the center diff will drain even more power that should be used up front to climb the wheels over the obstacle.

Sorry I can't make it less complicated, just try to visulize it in your mind.