In cave diving we have a saying, "Lights are life!"
You need to ask yourself a few important questions:
First, how much night diving am I planning to do?
Second, how much money am I capable or willing to allocate to my dive lights?
If you are going to be an avid night diver and cost is no issue, a primary light with a minimum of 10 watts should be chosen and a minimum of one, but preferably two smaller 5 to 8 watt backup lights. For this my general recommendation is the UK Light Cannon HID for use as the primary, two UK SL4 light as your backups, and a glow light (usually required by the dive boat captain).
If you are the occasional (once or twice a year) night diver, I could recommend any one of many halogen primary lights for use, a single UK SL4 light as your backup, and a glow light.
I have added some photos to help illustrate the recommendations. The last photo shows some lights (those big, bulky old UK 1200s) that I wouldn't recommend using anymore. They are old technology, heavy, bulky, costly to operate.
You should also add a "Dog Clip" to the light so you may attach it to your weight belt or BC (yes, I know, the little SL4 doesn't have one yet). If you, or anyone reading this would like to see how we attach lights to D-rings mounted on a tank to keep them within reach but out of the way, just let me know and I will post some mor photos.