Which has more stored heat - water at 100 degrees C or steam at 100 degrees C?

Steam, definitely. The latent heat of vaporization is inherent in the steam. To get from water at 100 C to Steam at 100 C, you need to add this heat. When you cool it down, it will condense and release that heat.

This is why chemical companies almost always use steam (usually pressurized because under pressure it holds even more heat) to heat their reactors.