Arrogant Safety Nanny
Join Date: Apr 2007
Location: Santa Maria, CA
Bikes: 2007 Trek 7.2 FX, 2008 Trek Madone 5.2
The 15w bulb was noticeably brighter. The 15w bulb was more of a spot beam, so it had a much brighter center and illuminated further down the road, but had less side spill than the 10w. I didn't really need the side spill of the 10w since I was using it in conjunction with the Solo 13w, which was already a fairly floody bulb.
I'd assume if you have a 10w bulb and a 15w bulb, both from the same manufacturer, with the same beam pattern, the 15 should put out noticeably more light. It's when different manufacturers start coming into play that things like color temperature, reflector quality/beam pattern, and other efficiency factors can start making the power consumption (watts) become a less reliable measure of perceived light output.