Working for SkyDemon, I see the split of guys who use "Wifi-only iPad + Bluetooth GPS" and "Cellular iPad + internal GPS" as being roughly equal.
Having a data SIM installed is not essential for the internal GPS to work, but you might find it nice to have if you need to get a quick briefing but are away from a wifi hotspot. Also, when you are in the air you will notice that SkyDemon can occasionally pick-up updated METARS and similar small data items when it gets a sniff of 3G coverage.