Cross training is much better. Mainly because it works different muscle groups and reduces your chance of injuring a muscle as a result, as well as giving the different groups down time to recover. For instance, running is very calf-centered while biking focuses on the quads. And when you compare running (high-impact) vs. biking and elliptical machines (low impact), you are also lowering your chance of damaging things besides the muscles, such as bones and tendons. I would, however, strongly encourage you to add some weight training to the mix, as too much cardio causes you to catabolize muscle. So a pair or three weight sessions a week will allow you to cardio to your heart's content without burning muscle. Also, it will allow you to build muscle, which eats calories, and therefore increases your base caloric needs per day, making it easier to lose/maintain your weight.
