AFAIK, they all do reverse motion. The big options that you have to consider are:
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Pre-programmed workouts, i.e. hills, cross-training, weight loss, etc. Some have more than others. Some may just have a manual adjustment where you adjust the speed and resistance.
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Arms that move or are stationary. If you want a little extra burn in the arms, consider the moveable arms.
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Heart-rate monitor. Some have a built-in heart-rate monitor. Some question their accuracy, but they are a good estimation.
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Adjustable cross-ramp. The cross ramp is the slanted plank that the foot pieces move up and down on. Some, like the one I bought, have stationary cross-ramps. Others have adjustable crossramps. By adjusting the cross-ramp, you can focus on certain areas in the lower bod, i.e. calves, thighs, gluts. I passed on this option, as I didn't like the way the adjustable cross-ramp model looked (too big and bulky) and I have a gym down the street, so I can challenge other areas of the body there.
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User Profiles: Some keep track of users profiles (i.e. weight, height, favourite pre-programmed workout, etc.).
I hope that helps!