The weight issue does not come from the differences in atomic weightm but in density of the material and which shape and tube wall thickness you can make for the particular mechanical properties. Steel has a huge range of compositions with vastly different properties, so a steel bike can be very light. On the other hand, steel can be welded and a cracked frame can indeed be repaired, not so for the carbon fiber. I think most people (including me) go for the feel of the bike, and it is great fun to test ride and see the differences.