I'm coming from a 4 so it'll probably be quite noticeable how much better iOS 7 will run on my 5S. Tbh I adored my iPhone 4 until the home button began playing up and more recently the power button has broke :-/ the annoying part is I looked after it, and didn't carelessly drop it or anything :-/
My 3GS is like brand new, not even a single scratch anywhere (without ever using a case, just a sleeve when inside the pocket).
I love my phone but it's gotten soooooo slow
256MB of Ram is too little nowadays, so I can't wait for the ultra slim and light 5S