Personally, I wouldn't upgrade if I had a 4. I've had the Siri app for around 2 years on my 3GS, and it gets boring really fast. And also, talking to your iPhone just sounds really cool for the first few months, but after a while, in normal working conditions and such, you just don't start talking to your iPhone.
Apart from the integration that Siri Assistant offers with 4S, it is EXACTLY the same as the Siri App. All Apple did was integrate it with the iOS. (Which kind of makes me mad, because they certainly took a long time to integrate the app into the iOS).
As for the speed... I think my 3GS is adequate in terms of processing power and speed. So I would definitely be very comfortable with the 4's speed and RAM. I guess the only real reason for upgrading to the 4S is the camera. But isn't the 4's 5MP camera simply amazing?
So all in all, I wouldn't upgrade to the 4S if I had a 4 because I see no real reason to. Unless you really crave for a speed bump and 1080p video recording, and talking to your iPhone.