A radiant barrier is designed to limit conductive heat passing through the roof into the attic space, and basically work during the "hot" months where Insulation reduces temperature "flow" from one space to another year round...winter and summer Barriers are often cheaper and if used with insulation can be very effective.

They are really two different animals, they were originally designed to be used together. The radiant barrier is you first line of defense it reflects heat energy and or cold transfer. The insulation act as a thermal barrier that slows the transfer of heat or cold from both sides of the insulation. So one reflects thermal energy and the other insulates or prevents.