Now that you've made the choice between iPhone 5S and 5C, what to do with your old iPhone? While you can always sell the phone, any profit is dependent on the quality of the phone. A phone that's seen its share of wear and tear likely has a low resale value. Consider turning that gently used iPhone 4S or 5 into the equivalent of an iPod Touch, perfect for kids wanting an iDevice to call their own.
To get started using an old iPhone as an iPod Touch, insert a SIM card into the phone - be sure the card is from the same wireless carrier that the phone uses. Connect the iPhone to a Internet-enabled computer with iTunes. The program will activate the iPhone, letting you use it as an iPod Touch.
If the SIM card used in the above activation is still active, you can remove it from the now iPod Touch and place it back in the device that will continue to be used an iPhone. Or, if the SIM card isn't activated, leaving it in the device is fine, since there's no paid plan connected to it. Make sure to enable a WiFi network, and you're good to use apps and listen to music as before!