I use two lights. The one on the handlebars is a garmin varia that adjusts with speed. It's pretty sweet but wasn't completely what I needed.
I also have a helmet mounted light. The one I have is a cheap explod-o-matic off amazon, so I won't recommend the brand as there is a possibility of burning down your house while charging it.
If I had to use one light, it would be a helmet mounted one. I like to be able to light up things where I look more than directly in front of the bike. Making a turn onto a pitch black road without seeing what is around that turn is not a comforting feeling.