If all goes well, the Captivate will be getting the software just as fast as the Nexus S thanks to the Devs. I think it really comes down to:
Nexus S: NFC, front facing camera, and LED flash
Captivate: Expandable storage
Really though, NFC is going to be pointless for a while and how often do people really need LED flash to take a picture at night (keep in mind that for the flash on most phones to have any effect, the subject has to be pretty darn close). I'd rather have a phone that will store Seasons 1 - 6 of the Sopranos
Oh, and with the Nexus S, you have to suffer through T-Mobile's spotty 3G network.