While the iphone 6 plus has a higher ppi than the ipad mini retina, it is is also a 5.5 inch device versus a 7.85 inch device. How much closer is one likely to hold a iphone 6 plus to their face? How far away is a person likely to hold an ipad mini retina to their face? Given the relative closeness in ppi(400 vs324) and more disparate screen size( 5.5 inches vs 7.85 inches), I would hazard a guess that to most people, the ipad mini retina may appear to be the sharper of the two devices. It may be fairly irrelevant, though, as at standard usage distance the ability to distinguish individual pixels should have long since dissappeared for most people.