Each has its own benefits
Morning,
I used to do the elliptical as my sole form of cardio at the gym - backwards, forwards, level 20, level 10, etc. I got a great workout!
As my leg problems increased, however, I found using the elliptical became more painful (I have a bone tumor on my right femur that juts out an inch from the bone, and really irritates the muscles). The up/down motion really started causing me problems.
Thus, I returned to the treadmill. Since that's walking on a more-or-less level surface, my workout problems disappeared. And because I could ramp it up to level 15 and keep it at a brisk pace for 53 minutes, I was able to get my heart rate up there and sweat out a storm.
After surgery recovery, I plan (hope plead wish for) to return to using the elliptical...but we'll see how things turn out.
I would recommend for you to try both and see what works best for your own particular condition.
Data points,
Barbara