I night ride offroad regularly, going out tonight in fact. I can't wait as its cold but dry here in my part of the UK, my favourite conditions (just got to watch out for the ice when it most likely freezes tonight!). I run a Solstice HID light (the lamp unit is actually made/sold in the US by Trailtech), it was a good price for a HID (£250 GBP) at least by UK prices.

I chose a HID after borrowing someone elses halogens for my first few night rides and hating the fact that you had to be careful which beam you had on when and for how long due to the paltry burn time (2 hours on lowish), our night rides often push the two hour mark and I don't like having to faff with things once we are underway - fit and forget for me. My HID has a five hour burn time on high (actually its only setting), this generally gets me a night ride and a commute (hour each way) without having to charge the battery in between. It also has a Lithium battery which is light and can be left on charge (for days - oops) without worrying if you are going to destroy it through overcharging. As it is ridiculously bright when I use it for commuting and it noticeably stops drivers pulling out on me, which is worth the price alone. I felt exposed commuting in the summer without it.

I have the light on the bars as our night riding is generally done on open moorland and rocky tracks, it does make getting round some corners interesting but that all adds to the fun. if I was regularly night riding on tight twisty trails, or trails with lots of hairpin bends etc then I would definitely want a helmet light as well (the HID lamp is a bit heavy for putting on my helmet) or as the main light.