I believe the Mars is brighter, but it is a single point light while the toplight has 3 LEDs that are focused a specific way. They are really 2 different things. The Toplight is powered by a dynohub and is on continuously while the Mars is usually in flasher mode (unless you want to replace batteries every week. Also, the Toplight spreads the light out laterally to give approaching traffic a nearness reference (as you get closer, the light appears wider). So says the marketing literature anyway.

I have to say though, that I have never had someone ride my bike with me approaching in a car from behind - I actually never have thought of it until now. Anyone here do this?

-G