Its part of it, but not the most important. Like more power, more light. but if you dont use quality products, like off-road lights from the dollar store, and you put 150w bulbs in them, they wouldn't be any where near as bright as a pair of quality 100w lights.
Then if you look at HIDs, 35w lights are brighter than a 130w light. Watts just tell you what kind of load your going to be putting on your electrical system. If you want bright lights look for lumens or candle power. More is better.