"Roamin Mike".....I'll give you what I think is a more practical view of the two. The exhaust brake is designed to basically block the exhaust and give the engine back pressure while at the same time, the Allison transmission is trying to downshift when it can. (The Allison will NOT allow your engine to over rev and damage the engine). If you aren't slowing sufficiently with the exhaust brake, the transmission won't downshift. The coach will continue to gain speed and you'll have to start pumping the foot brakes.
In the old days, the adage that you descend a mountain at the same speed that you climb doesn't really apply anymore. The modern diesels have much more power than before. My previous coach with an ISL 400, towing my 6K Silverado, could climb ANY grade on the west coast at 55+ mph. Even though it climbed well, the exhaust brake was not powerful enough to hold it back going downhill unless I was going really slow. My exhaust brake worked okay, but I typically had to drop below 44 mph to get into 3rd gear for the (exhaust brake) and trans to hold me at that speed. Anything above that, my coach would gain speed and required I stab the brakes often. This may sound okay, but I was getting run over by trucks and other RV's.
The exhaust brake worked fairly well when not towing and would have been better with a lighter toad. The only issue is that the exhaust brake is either ON or OFF. There is no adjustment for speed. This meant I did a lot of work while descending mountains to keep the coach at a reasonable and steady speed.
When I bought my new coach, an engine brake was a must have. My new coach is larger and heavier and I'm still towing my 6K truck. Now when I descend a grade, I can maintain 55-60 mph by using the two speed engine brake. Having two speeds really lets me descend at a faster speed without having to use the foot brakes. Anything under a 5% grade, I can usually maintain my speed using the engine brake on LOW. Above that, I have to use HIGH. A nice feature on my new coach, is that the cruise control will remain on and apply the engine brake to keep me within 3-5 mph of my set speed. I now just descend while the coach does all the work.
Soooo....with all that said, engine brakes typically become available as you move up the food chain of RV's. You're budget and RV choice will determine if you have an exhaust brake or engine brake. If you don't intend to tow, or plan on towing something lightweight, the exhaust brake might work fine for you. If you intend to do a lot of mountain driving and towing a medium to heavy toad, you'll want the engine brake if it's in your budget.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab