There is a lot of talk about what to use while trail riding, and I find that everyone has their own preferences that they swear by. I have ridden on trails with many combinations of lights so I will try to describe each set up the best I can.
Niterider Minewt 150 on Handlebar: This setup, with 150 lumens, provided ample light for fire roads, and slower single track type stuff. However, I found that with bmx style jumps, drops, really technical sections, and ridgeline descents, the 150 lumens was lacking. I have scene people bike with less light than this, but I find this to be a little scary as you approach a lot of obstacles rather blindly.
2 Niterider Minewt 150s on Handlebar: This set up allowed me to direct one light closer and the other further out. The advantage here was that the light shining closer up allowed me to see the terrain I was approaching, whereas the light shining further ahead gave me an idea of what to expect. This set up allowed me to run down the trail much faster. As the lights were mounted next to each other on separate sides of the handlebars, I also gained a little more spill light to the left and right of the trail.
Niteride 150 on handlebars with 600 lumen helmet mount light: I found this to be the best set up as my vision wasn't limited to where my handlebars were pointed. Because the bicycle will bounce around a lot, and you turn your bars to balance, or stay on trail, the light isn't always directed where it may be ideal for you with just a handlebar mount. Having a light on your head/helmet offers a much more steady beam of light pointed wherever you may need it. It becomes much easier to bike in the dark as you are no longer limited by and blindspots and unlit patches.
Some people may argue that the lower the lights are to the ground, the better your perception of bumps and obstacles become as the shadows are cast differently, and run multiple lights on their bars instead. While others I know only run a helmet light.
With all that said, it is up to you to decide what is right for you. If you are lucky your local bicycle shop may be able to help you decide, or you might be able to borrow some lights from them or a friend.
I can safely say that you do not want to go less than 150 lumens, though. The more the better. It may also benefit you to understand the throw pattern of the light you are purchasing. Some lights have an intense "spot" type beam, while others have a more diffused flood pattern. Once again, you are going to have see what is best for you.