Following to the PCWorld.com, here is the answer of “Why you must buy a new  iPhone 3G S?”

Faster Speed

Apple launched the iPhone 3G S by announcing it has a faster processor, more memory and “snappier performance”. Although the iPhone 3G is no slouch, it can be a little sluggish when opening and closing applications. If Apple’s demo videos are anything to go by, the iPhone 3G S will be a much zippier smartphone.

Big Size of Memory

Apple will sell two models of the iPhone 3G S. 16GB and 32GB.

Video recording

it’s cool! A smartphone in 2009 that can record video? No way! Apple critics will point to the fact that the iPhone 3G can’t record video as an example of a phone that lacks some basic features, but for potential iPhone owners the addition of video recording is a real plus.

New Camera

The iPhone 3G S has a 3-megapixel camera with autofocus, compared with the iPhone 3G’s 2-megapixel, non-autofocus camera. It still lacks a flash, but the increased megapixel count should slightly improve photos and tapping an area on the screen to focus on is a cool feature that should make mobile photography easier.

Voice Control

The iPhone 3G S introduces what Apple calls Voice Control, a voice recognition feature that allows you to make a call and play music by speaking. Voice Control can find any entry in your contacts list, and users simply have to say a name or phone number to make a call. For music, you can ask what song is playing and hear the iPhone 3G S answer, tell it to play your favourite album, or play similar tracks to the current one.

Compass

A built-in digital compass is another new feature of the iPhone 3G S. We aren’t sure exactly how often anyone would use this as a standalone app, but when combined with Google Maps, the compass will rotate maps to always match the direction you’re facing. Now that is pretty cool.

Internet Sharing

The iPhone 3G S can now be used as a modem, connecting to a Mac or PC via USB or Bluetooth. Although AT&T in the US won’t be supporting this feature, Optus is one Australian telco that has confirmed it will be. Telstra and Vodafone are yet to announce whether they will offer tethering capability.

More features

Apple has included a number of features that help people with disabilities use the iPhone 3G S. These include Apple’s VoiceOver function, which reads aloud what is on the screen, a zoom function that magnifies up to five times, and a white on black display option that provides increased contrast.

Nike + iPod integration

Previously only available for selected iPod nano models, Apple’s Nike + iPod is a fitness system that involves a Nike shoe communicating wirelessly with an iPod. The iPhone 3G S will be compatible with the system, which displays real time walking or running statistics.

Remote Headphones

The iPhone 3G S will include Apple’s remote headphones in the sales package. The new headphones have a multi-button remote and volume control keys, as well as a microphone to handle voice calls.