There is not really a difference, yes the 4s may be faster but, that's all. A hundred extra dollars for 1 second faster Internet and Siri? I say you buy iPhone 4 then buy iPhone 5 in 2012.
The major differences is that the 4S is much faster, has a better camera (8 megapixels compared to 4 which has only 5) and Siri, a virtual assistant.
I rather buy the iPhone 4 then waste my money on the 4S for a better camera and Siri. If I want a better camera, I'd get a real camera instead of 4S. Those cameras that costs a few hundred is even better than the 4S's camera.
If you want your iPhone 4 to be as fast as the 4S, you could just upgrade the OS, which doesn't cost a cent.