iPhone 4S GPS accuracy
The iPhone 4s, and the most recent iPhone 4 (Verizon) don't use a discrete GPS chip, as it's integrated into the baseband (Qualcomm MDM 6600). I believe the GPS chip in the iPhone 3GS used the Hammerhead II PMB 2540, or 2525 GPS chip. By integrating the GPS with the baseband it allows a smaller phone, less energy consumption, and cost less to produce. 

iPhone 4s GPS [Qualcomm MDM 6600 Gobi]

The main improvements should be:


Integration with Siri
Updated antenna switching (spatial diversity)
Improved TTFF and TTSF (fixes)
Longer battery life (more energy efficient)