I would wait until January 5th when the Motorola Olympus is rumored to announce.
The Olympus will be a dual core device so performance will be unbelieved fast and smooth this will be useful due to android 2.3 being based around improving gaming and the psp phone will bring PlayStation games to android
The nexus s will receive all updates first (which is a huge advantage in android ive been waiting months for recent updates) and is still a fast phone (1ghz) but this is nothing new to android devices so the only real advantage is the updates
They are both excellent phones and i am also interested in purchasing one.
So i would be happy owning either but am personally waiting to see the Olympus in action.