There are a number of different descenders on the market. Which one should you use and why?

Obviously there's no right answer to this question. It depends on your intended use. There are a lot of useful attributes that a descender can have. Most descenders will only be strong in a few of these attributes.

If you are doing wet canyons, the key attribute you probably want in a descender is quick release. The other attributes can still be important, but most are secondary.

If you are using the descender for general abseiling, particularly long descents, a balance of attributes is probably best. Heat dissipation, in line, variable speed and easy lock off are all useful to have.

The table below discusses what the important attributes are. Each of the commonly used canyoning descenders in the box to the right is discussed in more detail on their own page.

Safe A descender is a device in which you often trust your life. Most descenders have certain precautions that need to be taken, and in some cases equipment is used in a less than ideal way. These do not necessarily make the descender unsafe. However, I have tried to mention these safety issues in the discussion about each device so that you are aware of some of them.
Quick release In a canyon situation it is often important to be able to detach from the rope quickly. In general, if a descender can be attached to/detached from the rope without having to unlock the carabiner gate then it is much quicker. In addition, if the carabiner gate has to be unlocked, then there is a risk of dropping the device (which can be a major problem if you are swimming in a pool!)
In line Descenders where the rope runs straight through are normally preferable to those where the rope is twisted
Light weight The less weight you have to carrry the better
Variable speed It is an advantage to be able to change the speed of descent depending on the conditions of the abseil, and the gear you have to carry. Note that with practically any device, the rope can be clipped back through the main carabiner to slow the speed, so I have not mentioned this in individual descriptions. However this may reduce the ability to quickly release.
Variable speed during descent Some descenders allow you to change the speed of descent during the descent itself
Easy to lock off A descender which is easy to lock off is an advantage, as sometimes you need to stop and sort out tangles in the rope etc. You can often do a simple lock off by looping the rope around your leg or bum. However, if the knot or tangle is just below you, this is not convenient.
Rescue use Rarely important, but when it is, it may be vital. Some descenders are better suited to rescue use than others. Rescue use in this case refers to the need to carry an additional person down an abseil, eg as the result of them becoming unconscious.
Belay use If you need to set up a top belay, or someone needs to climb for an exit then some descenders can double as a belay device
Heat dissipation Descenders work through friction, and on long descents they heat up. With poor heat dissipation, the descender can heat up so much that it damages the rope. Heat dissipation improves with surface area. This usually means that larger/heavier devices are better heat dissipators, but some devices are designed to have a high surface area to weight ratio. Heat dissipation is only a minor issue in canyoning, assuming the rope is wet.