Disk or drum braking is a reverse displacement of horsepower into heat to slow down the moving object. I know this from fact because I once designed a braking system for a moving shaft of high amounts of inertia. I specified using a westinghouse air brake on the spinning shaft and the specifications called for to size it by horsepower. I knew my horse power of the motor that drove the spinning shaft was 400 HP. I sized the brake according to that horsepower with an additional duty factor. Worked like a charm.
Down shifting is using compression from the engine to slow down the moving object.
What is better?
Some measurables that come to mind to compare and evaluate slowing this moving object [vehicle] down are:
- Cost of engine slowing vehicle down.
- Cost of brake pads / rotors to slow down
- Cost of clutch assemble to slow down
- Safety
- Enjoyment
With all this being said I think one can plug and chug and make different decisions. I would think driving down a mountain item 1 and 4 would be more appropiate.
Driving sports car on the open highway at high speeds I would think items 1, 3 and 5 would apply.
Driving any car up to a light in traffic item 2 would apply.
My point is different situations require different methods.