the wifi chip on galaxy s is significantly better than that of nexus s, also the update 2.3.4 only support upto channel 11 so if your router is set on channel 13 then your phone won't connect to it...
nexus s screen is scratchable
Nexus s has only 16 gig of memory in which only 13gb is usable.
Battery life is worse than galaxy s since it uses LCD instead of OLED although you get better quality but not by much when OLED consumes less power compared to LCD...
Putting all these points together i would neither go with galaxy s or nexus s (i bought nexus s already but going to replace today or tomorrow) but instead i'm going with new galaxy s plus which has 1.4 ghz of beast cpu and it consumes 24% less power than galaxy s overall
