No difference. At the gym, a machine is a machine is a machine.
If elliptical machines are the triathletes of the workout room, treadmills are the Jabba the Hutts. A 220-pound person can use up to 787 watts of power on the treadmill, according to a test done by equipment giant Life Fitness. That's like turning on eight 100-watt incandescent bulbs for the length of the workout. Many ellipticals, on the other hand, rely on magnets, not electricity, for resistance; your hard work is all that's required to power up the machine's LED displays.
Skip the lights, TV, and air-conditioning altogether, and go for a walk or run outside.