I have used both of them and personally I don't like Samsung's TouchWiz.
I kindda like Vanilla Android.
Nexus S is by no means an outdated phones since dual-core smartphones won't make a difference any time sooner.
If you are gonna show the benchmarks and speed tests to your family and friends, get a Galaxy S II
Otherwise Nexus S, because it is a lot cheaper and a little thicker which, in my opinion, helps in handling.
I don't like the extreme thinness of Galaxy S II. I mean iPhone 4's thinness was, lets say, ok to handle but Galaxy S II is way more thin.
So if money is not a problem and you wanna boast in your social-circle and can bear TouchWiz, get the Galaxy S II
Otherwise, save money and go for Nexus S. You won't feel any difference in terms of speed and routine performance.
Can somebody answer this for me, What's so special about the SG2? (Except the hardware)
Design is another thing good but the TouchWiz and extreme thinness, in my opinion, keep it from being perfect.
It is still the most advanced smartphone you can buy