There are four or five boom brakes that are in common usage nowadays. I use a Dutchman Boom Brake, which is a bit oversized for my boat, but done because I was right at the limit for the one the next size down...and wanted to have the beefier unit because my boat doesn't really heel, so the pressure on the sail is much greater than it'd be on a monohull.
The Dutchman uses fixed and adjustable tension sheaves to create the friction used to slow the boom. It is the largest of the boom brakes that I know of.
The Wichard GybEasy is basically a friction brake that depends solely on line tension to create the tension looks like a bastardized rappelling carabiner.
The Scott Boomlock uses a tensioning line to control the friction exerted IIRC.
There's a Walder boom brake, but I've never seen one or used one, unlike the three previous devices.
The main reason I prefer a boom brake over a preventer is simple....a preventer allows the boom to load up a lot more significantly, and if it is released, can cause major damage or injury due to the stored energy in the preventer line. It can also cause a boat to be "pinned" by the mainsail in an accidental/unintentional gybe, knocking the boat down and leaving it there until the line is cut or released. Boom brakes allow the mainsail to still move, but reduce its speed to one that eliminates most chance of injury from the boom crossing over the boat. A boom brake doesn't allow the boom and mainsail to store up great amounts of energy, like the preventer can. A boom brake also isn't as likely to allow a situation where gear is damaged or the boom is broken.
Lets talk about boom brakes, they look like a great idea. Why not use them always? A simple piece of metal wrapped in a rope, and gybes are a lot less stressful. I have never seen one in person, or seen one used. Do they work? Are they hard to use? How about an adjustable shock cord on the mainsheet connection? It would pull the boom to center when the wind shifts, (or stalls), and slow it down when the boom goes over to the other side, just like a person would on a controlled gybe. An adjustment would control tension for light or heavy winds.