I definitely prefer helmet mount for off road and even road. When riding off road on tight twisty trails, a handle bar mount just doesn't work as good. You need to look where you want to go, not where you currently are, which is what you get with handlebar mount.

For road riding, either works, although I do prefer the helmet mount. I also like to give the cars a little flash when commuting, makes me feel safer. Also, for heavy fog, I find it good to be able to flash the cars.

You do need to get used to having the light on your helmet, it is additional weight on your head that your neck might feel after the ride a little. Once you do it a couple times though, you'll get used to it.