I am a former Vibrant owner who now has a Nexus S. This is the way I see it:
Pro Nexus S
-Back camera flash
-Little to no lag
-Fast Android updates from Google
-Not as must "bloatware", although Google installed as many Google apps as they could get their hands on
-TouchWiz (I find myself missing TouchWiz launcher and those minute changes Samsung made to improve upon Vanilla)
-Touchkey arrangements (I find the Nexus S key arrangements to be retarded)
-Extra $300 in my pocket
-Accessories are out
-Dealing with T-Mobile (for exchanges/problems) is a lot easier than BestBuy/Samsung
-Nexus S has better battery life. In my experience, battery life is about the same between both phones.
-Curved display is terrible. After owning the Nexus S for about a week, I can say curve display makes little to no difference; you may even forget it is there.
Final verdict: Extra $300 I paid for this phone (after selling my Vibrant)... worth it? Eh, I am still trying to figure that out. Is Nexus S a kick-ass phone? Hell yeah.