Apart from being insanely expensive, treadmills and elliptical machines are relatively the least effective for belly fat loss, weight loss and muscle-building.
Studies have shown that running on treadmills may be less effective than running outdoors.
This is based on various reasons such as stride inappropriateness on treadmills vs natural running, slightly less fat burn compared to outdoor running and other alternatives.
Changing-intensity exercises (such as walking, running or sprinting) are much more effective compared to repetitive and boring elliptical and treadmill workouts.
Changing-intensity exercises train your heart rate in a much broader range contrary to treadmills and elliptical that favor the same heart rate range throughout.
The following are 3 alternatives to treadmills and elliptical that can lead to a much faster belly fat loss, general weight loss and muscle-building.

1. Outdoor wind sprint exercises
This is the most appropriate workout for a rock hard, super-lean and ripped body.
Compare the strength-revealing physiques of professional sprinters with the 'sickly' physiques of professional marathoners.

2. Big upper and lower body, multi-joint, compound exercises
Bent over barbell rows, seated cable rows, one-arm dumbbell rows, bench press, pushups, dips, lat pulldowns, pullups, chinups, overhead dumbbell and barbell presses, barbell or kettlebell clean & presses, front squats, back squats, overhead squats, bodyweight squats, regular deadlifts, sumo deadlifts, Romanian deadlifts, single leg lunges, single leg step-ups, jump lunges,etc
3. Uphill sprinting
Running fast uphill as possible, followed by walking down and repeating as many times as you can for a full workout (very favorable for steaming up metabolism and fat loss)

As you'd observe when you try them, all the above 3 simple workout alternatives stimulate a fat-burning hormonal response and steam up your metabolism, which favors a more accelerated fat loss than treadmills and elliptical machines.