What is your budget for the light? Personally I prefer a helmet mount to a bar mount since I can shine the light in the direction I'm looking as opposed to the beam being pointed in the direction in which the bar is facing at the given moment.
I really dig my Niterider Lumina 650 as a helmet light as its plenty bright and cordless. I've got a Magicshine on the bars with a monster battery and Action LED Lights Wide Angle lens for a better pattern on the bars.
as stated previously, if only using one light go with a helmet mount. I've got more than one light set but often just use a Magicshine 808 on the helmet and it is more than enough light. http://www.action-led-lights.com/
I have a Jet lites A51 and a magic shine 808. The jet has the nicer helmet clamp so I use it on the lid.
You should have some type of backup light, even if its a bright flashlight, just in case your lamp dies.
When you spend more money on a premium headlight you are getting a better lamp but the money really goes into a better battery pack. I will say that I have had no issues with either of my lights or batteries. I sure as hell wouldn't spend more than about $250 on a "premium" brand light.