It is not the simulator itself that helps to lose weight, but the cardio load on it. I prefer walking on a treadmill more. Better to alternate. The effect will be anyway.
In principle, both cardio loads will be useful and help in the fight against excess weight. However, you can still burn more calories on the treadmill than on the stepper.
In addition, exercises on the stepper are monotonous, the movements are monotonous, which can quickly get bored: i.e. there is a risk that you will quickly abandon training on it.
The stepper works well for the calf muscles, buttocks, and thighs. However, a larger number of muscles are still involved on the treadmill, for example, even the back, abs, arms (although not like the legs) are turned on.
The treadmill is scolded by many for creating a shock load and hurting joints, spine (more when running, but when walking fast, there is also a load). The stepper is a little safer in this regard. However, please note: if there are problems with the knee joints, the stepper can aggravate them.
But if you choose a simulator for the home, then the stepper is definitely more convenient: it takes up much less space, it does not need to be connected to the network, and if necessary, you can even put the simulator into a closet. You can't do that with a treadmill.
This makes it easier to burn more calories on a treadmill, but overall the stepper will work well too, and if you eat the right diet, it will be easy to lose weight.
It doesn't really matter: a stepper, a treadmill, an exercise bike, an ellipsoid ...
It is not the simulator itself that helps to lose weight, but the cardio load it causes. Stock up on a heart rate monitor, calculate your "fat burning pulse" (220 minus age multiplied by 0.65) and get up or sit on any exercise machine .. Effective fat burning begins after 30 minutes of such a load, ie. workout should be at least 40 minutes.
But in my opinion, brisk walking or jogging in the fresh air is much more pleasant and useful.
P.S. and a toned body is a completely different story.