Whether and exercise bike or a treadmill is better depends on the individual user. Treadmills are better for losing weight and building overall muscle tone. However, stationary bikes are more easily stored, cheaper, safer and easier on the joints.
There are three things to consider when choosing home fitness equipment: effectiveness of workouts, long-term sustainability and safety. Treadmills are designed for weight bearing exercises such as walking and running and are the best for building bone density and overall muscle tone. They're also superior for developing cardiovascular fitness and losing weight. Studies show treadmills burn 750 calories per hour compared to 550 calories per hour for stationary bikes. In addition, they're more versatile in that they can be used for cross training, interval training, incline intervals and walking on toes or heels.
However, treadmills are also harder on joints than stationary bikes, and there's always the risk of falling off. Finally, they cost more and are harder to store.
Stationary bikes, while unable to provide whole body workouts like treadmills, are just as popular because they still provide great cardiovascular workouts with minimal stress on the back, hips, knees, ankles and feet. Physical therapists have used them for years to rehabilitate athletes with knee injuries. Another advantage is that they increase range of movement and flexibility, particularly in the hamstrings, which alleviates back pain caused by muscle strain. Finally, they're much safer than treadmills.