iphone 5: robust, tried and trusted operating system. Huge number of apps downloadable from app store. Great voice quality. Feels great to hold in the hand. Plenty of accessories available and in-car integration , if thats what you want.
iOS is guaranteed to be supported on each iteration of the IPhone for at least another 3/4 years so you'll keep on getting new software features for at least 3/4 years. I think only the original iphone (1) has stopped being supported on iOS just recently, nearly 6 years after its release.
Customisable to an extent out of the box, but even more so if you jailbreak, but the latest iOS version is not jailbreakable yet and probably wont be so for a good few months, if at all.
S4: plenty of gimmicks like the face movement detection thing, which doesnt work that well.
Slow, jerky operating system, but you can root and custom ROM your own if you are that way inclined for a better user experience.
Android versions generally only get supported for around a year, after which you'll have to rely on custom ROMs to see the latest Android features on your phone without going out and buying the latest model.
You can tell which one I prefer
