Simple, Treadmills are the most natural form of exercise because they replicate walking and running. Treadmills provide built- in motivation enabling you to set the tempo at which you wish to exercise. Treadmills replicate walking up hills or mountains with incline features. Treadmills naturally absorb shock, lowering the impact of the workout. You can also watch television or read.
Exercise bikes are non-weight bearing.
You can achieve all the benefits of an exercise program with less stress to knees and other joints. They take up little room relative to a treadmill. They are also easy to move around. You can read or watch television comfortably on an exercise bike. A good exercise bike monitors at least, speed, distance traveled, time and caloric burn.