Apple iPads come in either a wifi only version or a wifi + cellular enabled version. Your Phantom has GPS to know where it is and to set the home point for RTH, so you can fly your Phantom with either version. Some optional features require GPS at your RC (controller) so that your RC location is also known. Phones, cellular enabled iPads, and most Android tablets have GPS that can be used for those particular features. A data plan is not required if you go with a cellular enabled iPad.
Take a look at this high level overview of different options when using the GO app with an iPad. Forgive me if you have seen it before since I have posted it to the forum numerous (>50) times so you may have seen it before in your research. Please let me know if I have inadvertently misrepresented or omitted something.
[JAN 13, 2016] A new thing to consider is if you will want internet access in the field so that you can instantly unlock self authorization zones in the emerging GEO system from DJI. Without internet access in the field, you may need to plan ahead and unlock self authorization zones in advance if your country has GEO and you are impacted by one of the zones. This is similar to how you might plan ahead to cache maps in advance. We will understand more once GEO is implemented. For now, keep it in mind when considering which device to buy.
Wifi only iPad (no internet access in the field)
* Can fly your Phantom
* Can use maps if desired, but maps must be cached in advance
* No access to new maps in the field
* Can use Waypoints
* No LIVE STREAMING (this is different than FPV)
* No Follow Me
* No Dynamic Home Point
Wifi only iPad (tethered for internet)
* Can fly your Phantom
* Can access new maps in the field if desired
* No Follow Me
* No Dynamic Home Point
* Can use Waypoints
* Can use LIVE STREAMING, but likely not well.
Wifi only iPad with an external GPS
* Can fly your Phantom
* Can use maps if desired, but maps must be cached in advance
* No access to new maps in the field
* No LIVE STREAMING
* Can use Follow Me
* Can use Dynamic Home Point
* Can use Waypoints
* External GPS is more accurate
* Cell version not needed every time you upgrade to new iPad
* One more thing to charge and carry
Cellular iPad with no data plan
* Can fly your Phantom
* Can use maps if desired, but maps must be cached in advance
* No access to new maps in the field
* No LIVE STREAMING
* Can use Waypoints
* Can use Follow Me
* Can use Dynamic Home Point
Cellular iPad (no data plan, tethered for internet)
* Can fly your Phantom
* Can access new maps in the field if desired
* Can use LIVE STREAMING, but likely not well.
* Can use Follow Me
* Can use Waypoints
* Can use Dynamic Home Point
Cellular iPad (with data plan)
* Can fly your Phantom
* Can access new maps in the field if desired
* Can use LIVE STREAMING
* Can use Follow Me
* Can use Waypoints
* Can use Dynamic Home Point
Tethering your phone to a wifi only iPad does not share the phone's GPS unless you jailbreak. If you have a wifi only iPad already and don't want to jailbreak, you might consider an external (bluetooth) GPS device if you want to use the features that require GPS at the RC. There are many threads on the topic. It seems that the most talked about here on the forum are Bad Elf, Dual and the Garmin GLO. Just search and you will see plenty to read for research if you want to look into that option. [If this option interests you, one thing to watch for: iOS 8.3 broke the ability for an external GPS to transfer data. It was fixed with iOS 8.4 and later, and still works in iOS 8.2 and earlier. It is only iOS 8.3 with the issue.]
As noted earlier, Android tablets almost always have built in GPS so they are a great option to consider.
There are third party apps, so if you are interested in trying something other than the GO app you should check if what you want is available for iOS, Android, or both.
*Notes:
'Dynamic Home Point' is used here since that is a common way to refer to that functionality. Remember that the home point is not actually dynamically changing. You must manually change the static home point to a new static home point.
'LIVE STREAMING' is the feature that allows for posting your FPV to the internet in real time.
Tethering is connecting one device to another. In the context of mobile phones and tablets, tethering allows sharing the internet connection of a phone or tablet with other devices that may not be able to access the Internet directly.
RTH works with both wifi only and cellular enabled tablets and phones.
Maps are not required to fly, but many find them useful.
GPS alone does not transmit map/terrain data for you to download to your device. If you do not have internet access where you plan to fly and you want to use the maps, they must be downloaded (cached/stored) in advance via the internet. Most know this is obvious, but some new pilots may not realize this fact.