Galaxy S - Gorilla Glass, Super AMOLED, Rarely updated by Samsung.
Nexus S - W/O Gorilla Glass, W/O Super AMOLED, but backed 100% by Google.
You Decide. Personally I have Nexus S, SGS, SGS II, DHD, Senstaion & iPhone 4 & BB 9780 I simply love iPhone 4 & Nexus S cause I get the updates when they are out from the Manufacturer and the benefit of this is that the device is properly security patched, as I use official mails on my device.
If you are OK will rooting then undoubtedly go for galaxy S.