For what it's worth, I just finished the first part of cave training.
The Light Cannon is a great light, but as has been stated, not appropriate for caves. Although it will get you through cavern, becaue of its configuration and size, isn't really the right light for a backup; so ultimately, buying one would not serve your purpose.
Before deciding on my canister light, I did quite a bit of research. The one I finally chose is the Salvo 21 HID.
http://www.salvodiving.com/inc/pdetail?v=1&pid=2428
I also like the Green Force lights, which I have had occasion to use. I ultimately chose the Salvo lights (which I had some experience with as well) because my instructor also recommended them.
A big issue for me is quality. I'm certainly not going to mention qualms about specific companies' lights, but suffice to say I heard the most positive and least negatives about the Salvo and Green Force lights.
One final point, the canister light is critically important in a cave. For me, at least, saving a few hundred bucks but maybe winding up with something inferior is just not worth it. There is usually a reason one light costs twice with another lights costs.
As a simple example, the Salvo light can be lighted on shore and then turned off after it comes to full brightness. Some lights, as I understand it, are not capable of being ignited out of water. With others, striking them out of water means you have to let them burn for several minutes before turning them off. They get hot, and when you are gearing up, that extra few minutes can be annoying.
Jeff
