Even if I don't generally like the tone of John Gruber's Daring Fireball blog, his Larger iPhone Display Conjecture is well worth the read.
He guessed but got exactly right both the resolution in points and in pixels for both models, except that he did not (me neither) expect Apple to build a smaller resolution physical display and scale down (details are in @Tommy's answer).
The gist of it all is that one should stop thinking in terms of pixels and start thinking in terms of points (this has been the case for quite some time, it's not a recent invention) and resulting physical size of UI elements. In short, both new iPhone models improve in this regard as physically most elements remain the same size, you can just fit more of them on the screen (for each bigger screen you can fit more).
I'm just slightly disappointed they haven't kept mapping of internal resolution to actual screen resolution 1:1 for the bigger model.