Reasons to buy iPhone 4S:

Dual-core processor and 7x more powerful GPU
Siri natural language commands and dictation engine
8MP camera with improved low-light performance and capture speed
1080p video capture with stabilization
GSM/CDMA dual-mode device
No more death grip thanks to dual antenna design
64GB option
Power-efficient Bluetooth 4.0 connectivity

Reasons to stay with iPhone 4:

Same design
Same screen too, both in terms of size and resolution
The amount of RAM is the same
Siri doesn't work too well for people outside the US
You'll manage to snap more photos and longer videos and avoid 1080p playback problems