S=Hummingbird SOC,SGX540 GPU,WM8994 DAC so it has the best sound quality and the highest GPU power among single core phones.Has great developer support.

SL=Qualcomm MSM8255 SOC,Adreno 205 GPU,SCLCD display,so-so developer support.Nothing outstanding about this phone

S Plus=Same as SL but the CPU is overclocked to 1.4GHz but still,nothing outstanding about the phone.
I recommend the Nexus S,it's a Nexus device which means stock android that's fast and fluid plus fast updates from

In day to day use the difference in clock speed of the S and S Plus shouldn't be really noticeable but overall the S is the best among those three.That said i am not sure whether the GPS on the S is still problematic or not on the current firmware.

Still,if i were you i'd get the Nexus S.

go get nexus s bro..still among top 5 or top 10 fon from any reviewer...

Nexus S over Galaxy S
-It's a nexus device which will receive updates the fastest [the main draw of it imo]
-Stock android that's fast and fluid
-It has a LED flash
-It has no GPS problems

Galaxy S over Nexus S
-Arguably better looking design
-Has a microSD slot