The iPhone 3GS is the fastest, most feature rich model so far, and if you don't already have a iPhone 3G it's well worth the money. If you do have an iPhone 3G though, it's not quite so cut and dry. The 3GS may be is faster than the 3G but still it has its defects.
The first thing you'll notice about iPhone 3GS is how quickly you can launch applications. Web pages render in a fraction of the time, and you can view email attachments faster. Improved performance and updated 3D graphics deliver an incredible gaming experience, too. In fact, everything you do on iPhone 3GS is up to 2x faster and more responsive than iPhone 3G.