It's been a couple of years since I bought lights, so my comments are more general.

I like NiteRider and have had good results with both their products and their customer service. I like LED lights for their durability, light output, and battery life. On the MTB, I like running two lights: one on the bars and one on the helmet. Consider the weight of any light that you're thinking about mounting on your helmet. My TriNewt rocks, but it's nearly a pound and uncomfortable to wear on my head. I have a lighter one for my helmet.

Good lights cost money. I've probably got over $500 in my current set-up. For specific reviews and current product suggestions, I'd head over to forums.mtbr.com.

Night riding is a blast- I wish that there were more places that allowed it here!