I still have my old gold 5s on 7.12. I use it for other things now, battery life still great.
For many battery life on iOS 8 has been about the same as iOS 7.
I'm all for iOS 7.1.2, but it has its issues. There are some bugs, like every so often the photos app will force close. Of course, this version is recommended over anything 8, as the animations are very fluid in 7.1.2. There are also no stutters.
This version is NOT recommended over 8.1.3. 8.1.3 is fluid, good battery life, better functionality in a myriad of places, patched security issues, no stutters. etc. On a 5s, of course, even an iPhone 6 and iPad 2. On an iPad 2 runs same as 7.1.2.
I've read the complete opposite that iOS 8 drains battery on a 5s while on 7.1.2 has better battery life