If your gym is like mine as you walk in the door you are faced with a long line of various cardio equipment. The elliptical, treadmills, a variety of bikes and maybe a NuStep are the more popular.
New York Times reported on a study from the British Journal of Sports Medicine in which workouts on the elliptical and walking were compared. There is no one size fits all answer.
The elliptical uses the muscles of the hip and thigh more than walking. It also places a greater strain on the lower back. If you have hip and knee problems, the elliptical might be a better choice. Those new to exercise tend to find walking more manageable. I tend to have my participants try both to see what is more comfortable. One consideration for both is the use of inclines which can often make a big difference. However, the most interesting fact about this study for me was the impact both can have on bone density. Before you read on, which do you think is the winner is? The elliptical is less weight bearing (73%) than walking (112%). So if you are really trying to work on bone density, you know what you have to do if you can. Speed isn't always the answer either. Use the inclines on a treadmill or walk hills in your neighborhood for heart rate training and building bone density. The elliptical is always my favorite, but mixing in some workouts on the treadmill will pay off in the long run.
Resource: Exercise ETC. FitBits May 2014