For MBT at night, two lights are definately best - one on the bars and one on your helmet. The thing is: the trail may turn, but your handlebar light stays straight! The helmet light shines wherever you look, so you can follow the trail with your head. Night Rider HID lights up EVERYTHING because it's so stinkin' bright, so it's great for the bars. But, since HID can be too bright and wash out the trail, something not so bright (halogen) is better for the helmet (not so heavy on head either). Also when you look at your riding buddies you won't blind them like you would with HID ;-) Our night-riding MTB group typically doesn't use red flashers unless on pavement (bike path or road) or in danger of autos behind us - they can be pretty blinding out on the trail if there's a big group.
Have fun!!!!
~~~TrekChick