RedmondPie provides a quick hands on comparison between the Apple iPhone 3GS, Apple iPhone 4 and Apple iPhone 4S. Maybe the video which follows will help yo decide if the iPhone 4S is actually a worthy upgrade from the iPhone 4 or even an iPhone 3GS.
We personally feel this speed test is a little rushed and may not properly distinguish the performance between these three devices, but it may give you an idea of what you can expect in terms of general performance.
It is important to remember that the iPhone 4S performance improvements would be most effective in performance hungry applications like games and the 8MP camera has also not been compared with it's 5MP counter part in this video. The Siri voice assistant (which has been ported to the iPhone 4 through unofficial channels is one of the biggest selling points of the new iPhone 4S.