I dislike HID- they're unnecessarily bright for 95% of night mountain biking. My 15w Halogen is just fine, but the 180 lumen LED systems are even better. I borrowed a friend's L&M Stella for a few weeks, and loved it. You can't beat the 20,000 hour bulb life and incredible efficiency of LED's.
If I were in the market for a system now, I'd ONLY consider LED units.