I picked up a Nexus S recently as I couldn't resist the temptation to try out an official ICS build. People seem to rave about the Nexus S as well, with many former owners stating that they really miss it after selling it and upgrading to a different device. But in the end I really only wanted to keep one device, which put me in a position that is identical to that of the OP: choosing between the Nexus S and the Optimus 2x.
The Nexus S is a really nice looking phone. I actually appreciate it's curves and the way it feels in the hand, especially when making calls. Additionally, the location of the power button (right side of the phone rather than on top) means that waking the device up is more convenient. Audio output through the headphone jack is excellent. In these respects I feel it is clearly superior to the Optimus 2x.
In the end however, the Nexus S is short on both features and performance. I could actually live without the expandable memory, since it already has a decent amount. I could possibly even live with the slower performance. But the lack of (at least) 720p video capture, HDMI output and proper support for any sort of hw accelerated 1080p playback (which I admit may be kind of a redundant complaint given the former one) means 3 compromises too many. And it's a real shame too, because the Nexus S could easily have been my favourite little phone of all time, if only...
So yeah, I'm going to sell it and stick with my O2x. But I am also going to miss it. So I'm basically going to become one of those people I referenced at the beginning of this post.
As for ICS, while I would certainly choose it over GB, I have to say I was a little underwhelmed by it. It's nice, but I don't think it's quite the game changer that many people make it out to be.