At this late stage in the development of the iPhone 6, I'm not sure Apple would still be testing various display resolutions, but that's what the latest report on 9To5Mac is saying. According to the in-depth article, which cites sources familiar with at least one model being tested, Apple is considering a display resolution of 1704 x 960 pixels, which would give a pixel density of 416 ppi and an aspect ratio of 16:9. This compares to a resolution of 1136 x 640 on the iPhone 5s, with a pixel density of 326 ppi and the same aspect ratio of 16:9. The article explains in detail why this rumored resolution would be proportionately larger, making it easier for developers to scale up to the larger size.
The article gives the history of iPhone display sizes and resolution, and explains how the resolution has evolved. And while it acknowledges that the 1704 x 960 display being tested might not be the one we'll see, the article points to schematics leaked from iPhone manufacturer Foxconn that suggest this will indeed be the resolution of the iPhone 6.
Of course, the higher resolution means text, images, and videos will be significantly sharper on the forthcoming phone. The article suggests that this same resolution would also conveniently work on the 5.5-inch screen of the "phablet" that everyone is expecting Apple to come out with later this year.
The article includes some great mockups to show what iOS might look like on the iPhone 6, with its larger display and greater resolution.
So when when might we see the new iPhone 6? A recent rumor had said August, and that the 5.5-inch phablet would appear in September. However, the surest indicator (short of an actual announcement) is when Apple begins restricting when employees can take their vacation time. Apple wants all hands on deck around launch time, and AppleInsider is reporting that Apple is already restricting vacation time in September, suggesting that's when the announcement will come.