Galaxy S comes with 2.3.3 and Nexus S has recent update to 2.3.5. So software wise Nexus S will always be one step ahead. The reason why lot of members suggest Galaxy S is because Nexus S and Galaxy S has same hardware. Since they are same hardware wise, whatever Nexus S is capable of running must also run on Galaxy S. The difference between Nexus S and Galaxy S:
1) Galaxy S has Super Amoled and Nexus S has Super LCD
2) Galaxy S has no LED Flash and Nexus S has LED Flash
3) Galaxy S has 720p video recording and playback. Nexus S does not have 720p video recording
4) Galaxy S has FM Radio while Nexus S does not have FM Radio
5) Galaxy S has 16gb internal + option to expand external to upto 32gb for total of 48gb. While Nexus S only has 16gb Internal and no external sd slot
I have a Samsung Galaxy S variant (Captivate and Epic 4g) I am also currently running 2.3.5 thanks to CM7.