I agonised over this for a long time and ended up with a Schmidt. After my experience with a Schmidt, I suspect that the Shimano would have been more than sufficient.
I used this page to make my decision: http://www.myra-simon.com/bike/dynotest.html
I was convinced that I would feel the drag with the lights on, so wanted to turn the lights off when they were not needed and get the awesome advantage of the Schmidt. I never turn my lights off because I do not notice the drag. The power lost to driving the lights is less than what the slightest breeze will easily cause.
Check out the difference in speed due to lost power using this page: http://bikecalculator.com/veloMetric.html
For illustration, I entered some numbers that must be pretty close to me and my commuter, while I am cruising on my way to work (edited screenshot):
According to the dynotest page above, at 30km/h there is almost 8W drag from the Schmidt at this speed. It appears that the Shimano is more or less the same. For the same effort I go about 0.5km/h slower. Whoop-de-dooo.