Some people here have loved the elliptical machine. I didn't do so well with it. For me, the treadmill is much better. I actually injured my foot on the elliptical machine because you have to have some spine movement to use it, and I'm fused from T4 to Sacrum. Once I got going on the elliptical, I found that one heel was slamming down with every step because I just couldn't "go" with the machine. I also have osteoporosis, and I suspect that I got a tiny fracture. I never got it x-rayed, but from the amount of time it took to stop hurting, it probably was a tiny hairline fracture.
If you have access to a physical therapist, he/she could tell you what type of machine would be best for your particular back problem. Others here will probably chime in, too. Whichever you try, just start off carefully and listen to your body. I'm glad I got off the elliptical quickly and didn't do further damage.
Take care,
Emily