Mountain bikes: Downhill braking
Short answer: depends.

Long answer: 

Braking is as much a style as other parts of riding, but there are some general guidelines. The longer you hold those brakes, the more the heat will build up and braking performance will decrease. You'll also burn through pads a lot faster that way. On the other hand, sudden braking puts quite a bit of stress on the bike all the way from the rotor through the fork/rear triangle, to the handlebars and chain. It also changes the way the suspension moves through it's travel.

When should use either of these methods? I generally use a longer, controlled brake when descending a steep chute that requires a bit more technical riding or doesn't have a great runout after the rough bits. Quick braking is useful when you're plowing into a corner and need to scrub some speed quick or plan on drifting around it. You can also brake right before dropping in to a section so that the wheels roll over the rocks and roots instead of sliding and then roll out the bottom.

Really it's going to come down to how you ride and where you like to make speed changes. Experiment and find what's comfortable and what meets your needs. If you're trying to carry crazy speed you might find a quick, hard brake is more beneficial, etc.