Well, I think the treadmill would be the most effective, because you could run on it, and I find running is really great exercise for me. But, if you have knee problems, don't like running, etc. then I would recommend the elliptical. It is low impact which is really nice and easy on the joints. I don't think it's quite as intense as running, although you could certainly make it so by adjusting the settings. I also feel safer on an elliptical (I fell off a treadmill once, not pretty). Overall I would probably go with an elliptical if I were in your shoes, but it depends on your personal preferences.