I use a canister light, a 10 watt HID from Salvo. It's great, lasts 3.5 hours on a charge and because it mounts over your hand it frees your hands to do other things. The canister mounts on your waist belt on your webbing so it's out of the way. It has an adjustable beam: I normally use it at the narrowest setting (great for signalling my buddies -- they call it my light saber). However, when I'm using my camera I rarely use my canister light because it interferes with the shot (i.e. I get a yellowish light in the frame). Besides that, I have a bright LED focus light on top of my DSLR rig so I don't really need the HID. I don't use my canister light in the tropics: I find it's too bright for night diving.
One of my buddies uses a UK Light Cannon, and the difference is that: 1) you need to hold it so it uses up one hand entirely; 2) the light is more yellow than an LED and some other HIDs; 3) the beam is very wide and non-adjustable; 4) it's larger and heavier.
Another thing: where will you be using the light? Some of my friends have 21w and 24w canister lights, and if they use it in our local green waters and the viz is bad, the brighter light just reflects off the particles in the water. In this case it's better to have use a lower wattage (such as my 10w). However, if you and your dive buddies all have canister lights you should have the same wattage because otherwise the lower watt light will be harder to distinguish among the others, and signalling will be more difficult in daylight.
