The advantages of the S3 are:
- Shutter speeds all on one dial
- MUCH quieter shutter (the S-2 is loud)
- Parallax markings for your lenses (more accurate than you'd think)
- Finder allows you to shoot and frame a wide variety of lenses (markings for 35-50-105 but with a bit of practice you can easily frame 28-85-135 and, as stated above, finder field of view is 25mm)
- The original camera had a same level of quality control as a Nikon F, and I'm not aware of any user reports of S3-2000's having poor quality control. Many folks use them as daily users, moreso than the original S3's.
- The S3-2000's 50mm is among the best ever. A 1960s design, it has been tested alongside the most recent computer-assisted-design Leitz 50 and been found to have similar sharpness and excellent rendering.
Disadvantage - it's an eyeglass scratcher with wide angle lenses; some people do find the finder too busy (I always found the extra markings assist in rule-of-thirds composition, and allow you to very accurately frame other lenses.)
I've written elsewhere of how much I prefer the S3 finder over all others, but I'm very much in the minority (to me, it is THE ideal camera for shooting a 28mm lens through the giant life-size finder) however, because it is so hard on my eyeglasses, the past couple years I've mainly used an SP for convenience.