I love to cycle; I also do a lot of walking for pleasure and my out door personal training.. I could tell you first hand that cycling feels better, if you do not get hit by a car it is the best endurance sport out there. Cycling gets your heart, lungs and major muscle groups working to full capacity. The problem with cycling is that it is not weight bearing.
In a recent study published in Medicine & Science in Sports Exercise male competitive cyclists are more likely to have low bone densities in their spines than men who are moderately active. Thirty-two male competitive road cyclists and 30 physically active men were recruited in this study. Bone mineral density and dietary calcium intake were measured Resting serum and total testosterone was also measured. There were no differences between the cyclists and the active men in body mass or testosterone, cyclists did have lower bone mineral density in their spines, and were more likely have osteopenia and osteoporosis compared to the control group.
Take home message- Bone loading exercise is important even in competitive athletes.