W, cheapest of all, performance on par with S+ but it's released the latest, thus lack of "community support"

S+, same chipset thus should have same performance but it has a bigger screen, a slimmer profile and AMOLED but resolution is still the same. That means pixel density is lower than that of W. More stylish than W, lose on price and LED flash for camera. It has gorilla glass too.

Nexus S, quite old, Ori is not worth buying, AP Nexus S is priced in between Ori W and S+. 1GHz stock, GPU is SGX540 which I think is more powerful than Adreno 205 seeing how Galaxy Tab perform on this GPU. The thing here is... If you're to get Nexus S, believe me, root it and you'll be amazed. My friend got his nexus S rooted and it really get optimized. Since Nexus S is branded as "Google", it's widely supported by the community, a lot of developers released ROM and kernel for this. He flashes ROM like... every week, so much fun to play there. But you need to know that you can't get AMOLED version of Nexus S now, except 2nd hand. You'll get a SLCD here in Malaysia. The nice thing of SLCD is there's more subpixel compared to AMOLED due to what what pentile thing, google for that. Nexus S, if rooted, and overclocked, is capable of having much better battery life than stock and also unleashes its true power. OC to 1.4GHz, quadrant is higher than stock S2. Depends on which ROM you choose too. Really amazing what that device is capable of doing. Appearance wise, it's quite thick but quite nice to hold due to the contour design. Last thing, Nexus S got NFC. Might be useful in far future. (Other countries I would say Near Future =P)

Oh ya, Nexus S shall get official ICS upgrade. W may not be getting it.
All 3 are made by Samsung, shouldn't be much difference in built quality.

Edit: Forgot to mention the built-in storage. Nexus S got 16GB, S+ 8GB, W 4GB... erm... or should i say 1.7GB. Lol...

Edit2: Nexus S no 720p recording~ Forgot to mention too.