First, the announced resolutions:
iPhone 6: 750x1334 326ppi 4.7"
iPhone 6 Plus: 1080x1920 401ppi 5.5"
I think that means iPhone 6 will use @2x image assets resulting in screen coordinates of 375x667 "points" (vs. 320x568 for current 4" iPhone screens and breaking from the 320 point width of all existing iPhones).
I suspect iPhone 6 Plus will use @3x assets resulting in screen coordinates of 360x640 "points"
That gives both phones a 16:9 aspect ratio.