You're close but not quite there.
IPads are similar to Androids, but most agree that iPads are better designed and better made. The screen is sharper, brighter, and the touch mechanism works better, etc. Androids are simpler and cheaper and some say they have certain advantages. And the two products have a complete different operating system. Hence, software for one won't work for the other.
IPads don't need cell service to get GPS. Both products (provided the specific Android has GPS capability built in) have to have GPS capability, and then they also have to have a GPS app of some sort.
The advantage of having a device that gets cell service is that when you're flying around with your Phantom app and look at the satellite image of the area your flying in, your cell-service device can bring that photo to you via the nearest cell tower. If you don't have that cell service, you're going to get a blank map unless you thought ahead. You're going to have to have the map already loaded into your device before you fly because you're already using your wifi to communicate with the Phantom, aren't you? And besides, you likely won't have wifi in your flying area, unless you are flying right outside your house or you happen to be right outside a Starbucks or something. But if you did have an active wifi connection, you'd still have to effectively download the photo maps manually before you fly. Your device can't connect to two wifi connections at the same time.
In summary, it's best to have cell service on your device. It's easier. But it's not "needed."
Hope this helps.