Well, since you are asking on an iPhone forum on a site specialized in Apple news, you should pretty much know what people will say around here: go for the iPhone.
However, the answer depends on you and how you use your smartphone. There are a lot of questions for you to answer in order for you to find out which is better for you, but when I suggest a new phone to a friend, I normally follor these guidelines:
Go for the Android - in your case the S4 if:
- you like tweaking with electronic devices and have time to work your phone to meet your needs;
- if you don't mind doing your own security in your devices - running antivirus scans, setting up anti theft solutions, etc;
- you had Android devices before and want to keep your purchases (musics, apps, etc.) in the transition
Go for the iOS - iPhone 5s if:
- you don't have the time to make your phone meet your needs perfectly and just want something that 'just works';
- you had iOS devices before and want to keep your purchases (musics, apps, etc.) in the transition
With these guidelines I normally succeed in convincing my friends into buying the right phone for them, and so far everyone is happy with their purchase. I hope this helps
EDIT - wrong button pressed when previewing...