iPhone 3GS repair or sell ?

Hey

I have an iPhone 3GS in white with 16GB. It has a lot of wear but it is still functional. The lock button is defect (I used it with the Assive Touch, so I can look the phone...) and it has many cracks on the backcover and the screen is scratched ... it pays to repair the iPhone or not? I've heard the Apple iPhone 3GS to iPhone 4 exchange? Sry for my bad english (I'm from Germany)