Back to the Mark Gurman 9to5Mac well again. Yesterday it was iPad split screens, and today it appears he has some scoop on what the screen resolution might be on the iPhone 6. In fact, he's speculating that both the rumoured 4.7" and 5.5" iPhones will have screen resolutions of 1704 x 960, resulting in PPI's of 416 and 356, respectively. I guess that makes my prediction of a 1420 x 800 display for a 4.7" iPhone unlikely.
This rumour, although still just a rumour, makes more sense to me than my spitballed prediction. It's consistent with how Apple handled the last two display resolution changes with the iPhone 4 and iPhone 5, and it would make it possible to have both the 4.7" and 5.5" have the same resolution, while maintaining higher PPI numbers than the current iPhone lineup (326 PPI).