A lot of folks are confused about the need for 3G or 4G. All charts and data are downloaded to your device, preferably over Wifi, *prior* to takeoff. ForeFlight recommends that you turn *off* 3G/4G/Cellular Data and Wifi during flight. There are 2 reasons to choose the 3G/4G version over the Wifi-only version of iPad:
1) The 3G/4G-model includes the internal GPS chip, however, the Wifi-only does not include the internal GPS chip. The Moving Map and Geo-referencing require the internal GPS chip (ie 3G-model) or an external GPS.
2) To use the 3G/4G/Cellular Data feature to access the internet where there is no Wifi.
BTW, you can activate the 3G/4G feature and change your plan at any time. There is no contract for 3G/4G service. 250MB per month should be adequate for most ForeFlight Mobile users.