stinggirl
Lights are not the easiest choice anymore
Reliability - before buying a light do a quick search to see if the are an unusual amount of failures. There are some manufacturers that are known for amazing customer service... I prefer a reliable light that never has to be sent back.
Power/Lumens - More is usually better.
Watt - (W) is a measure of energy conversion, not brightness (50W halogen and a 10W HID can be similarly bright)
lumens - a measure of the perceived power of light (total light power emitted)
lux - a measure of the intensity of light hitting a surface (spot lights have higher lux than an equal Watt/lumen flood light)
Lamp/Bulb
Halogen - once great, but now more of a the 'budget' solution
HID - little better 'color temp' than LED, good burn time
LED - the new kid, better burn time, durable, probably the 'future'
Burn Time - Again, more is better. Consider how many dives you can get off one set of batteries or full charge
Beam Focus
adjustable - flexibility is nice, especially if you only have one good light
narrow - generally better distance, and ability to 'signal' your buddy
wide - lights up a larger area, rather than a 'spot'
Batteries
common size - some battery sizes are hard or impossible to find (especially abroad)
rechargeable - better IF you dive a lot and remember to recharge/maintain the batteries (outlet issues when traveling)
On/Off
switch - easier to use single handed, but one more place to leak
twist - two hands needed, will leak/flood if twisted too far open
Size & Shape/Style
handheld - common, usually smaller and easier to stow or attach (4AAA - 3C)
lantern - uncommon, handle above light, usually a larger (8D)
pistol grip - common, medium sized lights (4C - 8C)
canister - "technical", battery pack with 3' cord and a light head
Deciding
- determine the best 'lamp' for your diving (LED, 50 lumens, 20* beam)
- determine duration needed (2 x 1hour dive x 3 day trip)
- make sure the product is reliable and has a solid/easy warranty
- get a good 'backup' light, because even the best lights eventually fail