You technically have both, although the "exhaust" brake is actually a VGT or sliding turbo vane. It uses changes in the turbo geometry to create back pressure the way a traditional exhaust or Jake brake does. The Allison transmission controller also will downshift more aggressively to help slow things. 

One other note, when you have the engine brake switch on and it is activated (decelerating) it turns your brake lights on even if you aren't using the brakes. 

j