If both bikes or otherwise identical and weight the same, the carbon bike will have greater mass. This is true because you need to consider the effect of buoyancy. Carbon composite is less dense then steel so will have greater atmospheric buoyancy. So if the two bike weigh the same at normal sea level atmospheric conditions, the carbon bike will weigh more than the steel bike in a vacuum, or at high altitude where the air is less dense. If they weight the same at the bottom of the hill, the carbon will weigh more than the steel at the top of the hill.