Are you mountain biking or road riding?

If you're riding on the road, I would say the 400L would be better suited for use than the 800L. If you really want a lot of light on the road, I would still say 2 400L's are better than one 800L because they let you change out lenses to get the light spread you prefer (I prefer narrow so it goes a long ways down the road). I would certainly recommend a 400L and a 400L on the helmet before recommending just an 800L.

On the other hand, just yesterday I wrote a big piece on how I thought the shaped beam of the Lumotec/Bush and Mueller lights was far superior to the light output of even 2 400L's, having now had experience with both...