Front brake provides most of the stopping power. The harder you brake, the more stopping power comes from the front brake. For this reason, some will tell you to use front brake only.
However, if you're riding on a bad or a wet road, you don't want to brake too hard. Apply both brakes smoothly and gradually. Braking too hard or too suddenly can cause one of the wheels to lock, and then you'll crash. You particularly don't want to lock the front wheel when going downhill, because there's a risk of flying over the handlebars.

As long as I'm unsure of the road directly ahead (there's a risk that I might need to brake or turn, because of a car appearing out of a driveway, etc.), the hoods is the only place I'd be when descending, at any speed, 0 to 40 mph.