Should I upgrade the iPhone 4S?

I have the iPhone 4S for at&t and I can unlock it soon, should I upgrade it to iOS 7? Or just leave it with iOS6? Which would be better for resale?

You can update to IOS 7.It has really brilliant features added and looks. Did you see the Specs, features and price? If not http://webmalaya.com/new-i-phone-5s-and-5c-is-here/ you can have a look