I am a big fan of the elliptical machines at my gym but I was wondering what the benefits of using the ones that move your arms vs. the ones that focus on your legs only.

I am interested in loosing overall body fat so I want to know if one is better than the other?

The machines which include your arms are better as you theoretically burn more calories as your also exercising the upper body too. If you have a choice take the cross trainer which includes the arms. If you exercise on these correctly you can actually burn approx 25% more calories than on a trainer which does not include arms!