What is the difference between iPhone 3GS and iPhone 4S?

I have an iphone 3GS and the only difference is that you can take videos, you can use voice control, and it has more memory.

Let me improve that answer.

The iphone 3G[S] is more than 2x faster, has more powerful processors. (You need it for most 3D apps (like games) to run smoothly, and for augmented reality.)

The iphone 3G and original iphone CAN record video, once jailbroken. (This is free)

The iphone 3G[S] costs more than the iphone 3G.

The iphone 3G[S] has a digital compass. (This allows for navigation and augmented reality apps.)

The iphone 3G[S] is available in Black or White, while the iphone 3G is available in only Black.

The iphone 3G[S] has restrictions made by Apple to make jailbreaking just that little bit more complex, but it is always possible. When the 3G[S] was first released, jailbreaks for each firmware version were a while behind jailbreaks for the 3G, but now there is more known about its inner workings, and it is more widely adopted, jailbreaks come out at nearly the same time, if not sooner, for the 3G[S], so no worries now.

But if you are thinking of buying an iphone, I would STRONGLY RECOMMEND waiting till June/July to see the new iphone, and if it will be worth its price. There are reports of the NEW iphone being cheaper.