As the subject says, I will soon be upgrading from a 16GB 4S and both the 32GB 5S and 16GB 6 are the same price. Here is what I will be using the phone for mostly. Primarily KNFB reader, playing audio games, streaming audio/video as well as recording audio/video. I am thinking the 5S should probably be my choice because of the added space, smaller size and from what I have heard KNFB reader works quite well since I assume that's what it was mostly tested with before release.
I've gotten to see the 6 and I think it's pretty large but not that bad I suppose. The only thing that would change my mind is if the 6 has a huge increase in battery life over the 5S with brightness set to 0, screen curtain on, Bluetooth off and WiFi on.