15 watts will get you by while descending. You can go faster with a brighter light but fun can still be had with a 15 watter. 10 watts will do fine for climbing. It's good to be able to switch back & forth between the two.
With Sparty, I run two lights with one on the helmet and one on the bar. Both are adjustable wattages, I think they range from 12-20, and usually use the middle setting to preserve battery life.
Almost everyone I ride with seems to have the HID lights, they have several things going for them one of which is battery time. My lights will get 2hrs, maybe 2 1/2 if I am good, most HID's will get upwards of 4 hours, my wife's new light get's 10-12 hours of burn time.
I have as much fun as the next guy, and go just as fast if not faster
than most of them.
We used to all ride with 10watts kind of being the bright light, 15 watts was amazing then the 20 watt came out and that was incredible. Now we have HID's, next we will be carrying the Sun on our heads
Like I said above, I run two lights, I like the helmet for looking where I am going and the bar light for lighting the contrast. If I had just one I would run the helmet.
Hope that helps,