How obsolete is an iPhone 5?

 I've been thinking about replacing my old iPhone 4S, which has a failing battery and other problems, but really don't need anything really advanced to replace it. The most obvious choice would be an iPhone 5S; as small as the 4S, a bit faster, and affordable - you can get one unlocked for around £70. The cheapest 6 is over £100. The apps I use work well enough on the 4S though a couple are a bit slow, and I gather the 5 is a bit faster.

Any good reasons (other than ooooh! Shiny!) why a 5S isn't a good idea?

Before anyone asks, I don't want to look at alternatives to Apple at this point - some of the apps I like don't work on anything else.