Treadmills and elliptical machines are popular and highly effective ways to work out. There are some advantages in choosing one over the other though. An elliptical machine is less stressful on your knees, hips and back than running on a treadmill. However, walking on a treadmill exerts about the same pressure on your body as using an elliptical machine.
One advantage of elliptical machines is that they allow you to work out your upper body as well as your lower body. Ellipticals can also be pedaled in reverse, allowing you to work additional muscles. If you are training for a road race, a treadmill is probably the better way to exercise. Whichever you choose, both are excellent ways to work out if used properly.