I ride with a Nite Rider Evolution for both road and mountain (~$130). It gives off enough light for me to be comfortable. I just couldn't fork over the cash for a HID without knowing how serious I would be about night riding (which I ended up loving). I've been really satisfied with it and def recommend it. It can be mounted on the handle bars or the helmet, I prefer the helmet mount. While on the trail, it's obvious when someone has a HID, they're super bright, but for me, the halogen gets the job done.

Also, in case you don't already have one, be sure to get a rear light. They're pretty cheap and worth the money. You def need it for road riding and for mountain biking I found that many of the parks with the trails are closed so I had to ride a little distance on the road. I felt much safer with the rear light on for the short ride to the trail. For rear lights, I have two different kinds. One is a Nite Rider and has 3 LEDs in a row. You can set it to blink, stay on, or each LED blink seperately. It's a surprisingly bright light that can be seen from a decent distance. I like this light a lot and highly recommend it. The other light I got from REI and I do not like it at all. It was late, the lbs was closed and I was about to go for a trail ride and wanted a taillight. It's a Blackburn Mars 2.0 and a total piece of crap. It's big and bulky and doesn't give off even close to what the other light does. Lesson learned on this....just because it's bigger, doens't mean it's better