I thought I was going to be in the 5s club but man, once you get the 6 Plus, there is no going back. The 5s looks so small and the screen very dated compared to the 6 and 6 Plus. The extra contrast ratio and curved glass makes the 6 really pop. The 5 really shrinks once you use the 6 or Plus for a couple of weeks. It's crazy.
I have a 6. When I pick up my wife's 5S or my old 4S, they really do feel small -- but they feel great, like a true portable device that feels nice in your hand, especially when you have a good grippy case on them. While the 6 feels like it's trying to be more of a tablet-like device that happens to fit in your pocket. The 5S is the phone you want to use when you are on the go -- pull it out, do what you need to and then put it back in your pocket. You're not going to worry about it falling out of your hand, so you'll feel comfortable using it in a lot more situations. With the 6, you'll want to set it down a table to use if you can. I bet this even more exaggerated with the 6+.
The screen on the 6/6+ are really an improvement, though. Photos and video look excellent. I mentioned camera improvements as a reason to stick with the 6, but many photos taken with my 4S look just as good as those taken with the 6, when I view them on my 6.