I have an iPad 2 3G on AT&T. Currently, I toggle back and forth between WiFi and the 3G service. If I know there is a WiFi network, I go into settings and turn off the 3G service to ensure that I am not using any of my monthly data quota. Is this necessary? If I always leave both turned on, will the iPad automatically switch back and forth, and default to using the WiFi until it's unavailable, then use the 3G network, then switch back to WiFi if I re-enter an access point? If I leave both turned on, I can't tell which one it is using. Does it use both? Thanks.