There's no brilliant magic golden ratio formula behind Apple choosing 1080p. They simply wanted to choose a "regular" resolution for once, while still being bigger than 326 PPI.
If 720p would've been more than 326 PPI for the 4.7" one, they would've chosen that, too. The reason they didn't is because 720p is less than 326 PPI, and they didn't want to have 1,000 headlines out there say how "iPhone 6 has less pixel density than previous iPhones".
In my opinion they should've done that anyway, and end it with the stupid resolutions that Apple keeps using just because they didn't have the foresight to add resolution scaling early on to iOS, because they never imaged they could be building any other mobile device that isn't 3.5" in size. They messed up the "perfect scaling" for developers with 1080p on the Plus anyway. They could've done it on the smaller one, too. It's silly that they didn't do it.