In the rare times that I want to use the Phantom features which require GPS at the RC, I use an older Dual that I already had before my Phantom days. I simply strap it to my arm or hang it from my belt and it works perfectly. It has already paid for itself since I have not needed to buy the more expensive cellular version when I've upgraded my iPads.
Even so, since I don't care much about Follow Me or changing my home point to the RC location, my Dual sits in a drawer most of time. If you use those features often, bake into your decision the fact that an external GPS is one more thing to charge and carry. Perhaps a GPS enabled tablet, be it iOS or Android, would make more sense from a convenience perspective for a frequent user of those features.
One more thing to think about is the upcoming GEO system. From what I understand, having an internet connected device with built in GPS may be much simpler to use when you need to self authorize a flight location while out in the field. We are expecting to be able to self authorize at home in advance, but you'd need to know about it or plan ahead. Tethering your phone to an iPad does enable an internet connection for maps if desired, but since tethering to an iPad does not transmit GPS data, I dont know if it will allow self authorizing in the field to function in GEO. We need to know more once GEO is launched, though it does seem to be in a holding pattern. Keep this in mind as well when deciding.
