Sometimes I am going to fly my phantom with an iPhone, and sometimes with an iPad. If you make a change to the drone using one device, will it trasnfer to the other device as well?

The only bits that will need syncing are the "Flight Logs". make sure yuo sync them ( see iamge ) make sure to do this or when yuo use a device with no flight records and that syncs, you will loose what was done on the other.. Phantom 3 settings are saved to the craft itself. Only things that will not transfer like this are images/vide you save on the idevice.

Remember to buy extra propellers and one or two spare batteries to keep you flying