Desire S is a good model to get. I would recommend Nexus S because of these main points:
1. Vanilla android is better because HTC sense is real memory hog in Desire S or other HTC models.
2. Nexus S will be the first model to get ICS update. People do not understand and argue Nexus prime gets it first. It is first model to ship with ICS while Nexus S will get the update first OTA. So you will get the next major os upgrade without any hassles. That cannot be said for Desire S.
3. Nexus has galaxy hardware which is powerful than any other single core in the market. True, Galaxy S is one year old but it was times ahead when it was released last year. Still better than desire S, hands down.
4. Wolffson chip in Nexus S and Galaxy S which provides unbelievable audio performance.
5. Incredible xda community support for galaxy s and nexus s.
6. Nexus s has nfc chip and gyroscope so it is not found in desire s.
7. Gaming is still best in Nexus S and galaxy S. In fact best in the market for any single core phones. Sometimes even comparable to dual core.
So I would recommend Nexus S, hands down. Next I would choose Galaxy S and lastly Desire S. That is IMHO.