The 4s comes with a 1 Ghz dual-core processor, whereas the s3 comes with a astonishing 1.4 Ghz quad-core processor, making it far better for perfomance.
The 4s has a rather small screen at 3.5 inches compared to the s3's gigantic 4.8 inch screen. The 4s's glass screen would break with literally one drop to hard ground, whereas the s3 can resist this several more times with its revolutionary 'corning gorilla glass 2' screen.
The only thing which makes the 4s better is the sharpness of its display. It has a retina display at a very sharp 330 pixel per inch, whereas the s3 has a 306 pixels per inch display.
Overall though, as you might have figured out, the s3 is better. If you were to buy an iPhone, wait for the iPhone 5 which will be released sometime in autumn this year. May I inform you that the predecessor of the s3, the s2, was already good enough against the 4s. Therefore the s3 definitely hammers the 4s.