Neither is really considered better, they are both good choices. An elliptical is a great choice as it is considered to be the lowest impact on your lower body joints since your feet don’t move off the foot pedals. No heel-toe striking.  Biking fits in that category also, and many seniors enjoy a recumbent bike because it allows your feet to be out in front of you, putting even less body weight pressure on the lower body joints. A treadmill is great for walking, which is also considered low impact.