On ellipticals: We had one for several years, andI used it off an on. My biggest frustration was that it worked entirely different muscle groups than running, so most benefits didn't feel like they carried over to fitness tests (running) or soccer (running and agility). We ended up replacing it with a treadmill.
On treadmills: We had one for about five years, and it didn't see much use. I don't particularly care for running, as a rule: it doesn't engage my mind enough, and I get bored. Putting the treadmill in front of the TV or using an audiobook helped, but in the end, it was knee trouble that made the treadmill unworkable. As exercise for weight loss (my goal, although I've since learned that diet is significantly more impactful), it was ... okay. It was heavy, didn't move well, didn't store well, and after several years needed some maintenance, which although it was supposed to be super easy became one more thing on a long to-do list that never got done. We sold it when we moved. (The knee pain, which had grown progressively worse over a decade, was solved by a good foam roller and a chiropractor trained in Active Release Therapy. We also moved somewhere where outside running was possible for more of the year.)
Currently, we have a "Total Gym", which I rather enjoy, and have used in spurts for two or three years. It significantly altered the structure of my upper body, defining my shoulders, neck, and bulking my arms. It includes some of the same motions as a rowing machine, and is much more versatile. I recommend it.
We also recently picked up a "Bowflex Max Trainer", which is something of a cross between an elliptical and stair stepper. The muscle groups worked feel a little closer to running than a typical elliptical, with low(er) impact on the knee. In addition, Bowflex promises it burns about twice (or more!) the calories of a standard elliptical.
Bottom line, though, as JTJ said: The one you use the most.
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