In many (most) jurisdictions in the US, at least, you need both for night (between legal dusk to dawn) riding. My state requires a red reflector for legal night riding. Additionally, if you live in a state with a contributory negligence clause (I do), lack of a reflector may affect your ability (or that, $DEITY-forbid!, of your heirs) to sue/collect insurance/get authorities to enforce their reckless driving/vehicular manslaughter/injury laws if you are struck by a vehicle. I don't depend on a reflector (my daughter says my rolling light show is stupid-obnoxious bright), but my commuter bike has one to keep the lawyers happy.