Having recently upgraded my lighting from a 10 year old 6 watt system, I suggest that it is remarkably important to think about how, specifically, you'll be using the light, what you want it to do and under what circumstances. For example, battery run time for some people is very important. Something with a run time of 1.75 hours just isn't enough if you've got 2.5 hours of riding ahead of you. Additionally, there is a big difference between charge times for some systems. This can be an issue if you commute distances to and from work. You may need more time for a full charge than your work day provides.

The speed at which you ride, as suggested by others, is also an important factor. If you ride at 20mph, you'll need a pretty strong beam. Riding at 12 mph you can get away with less power. If you ride trails, that usually requires more light at the same speed than on residential streets that are lit. Additionally, there seems to be a difference between the shadow and three dimensional detail you can easily see with halogen vs. LED lights.

I know in my search I though I had it all figured out only to find that the "ideal" lighting system was less than imagined because the clamping system seemed flimsy and was hard to put on and take off (I needed to switch the light back and forth between different bikes.) Given the expense of high powered systems and the range of costs, I found it important to take a few weeks to really work through all of these things. I'd make one final recommendation. Consider purchasing your system from a seller who will let you return it, no questions asked, if you find it's not meeting your needs.

Oh, yes, and I ended up with the Cygolite Dualcross 200, because it met my needs pretty well.