I just started looking at the Galaxy S today and this phone looks like a beast! I am one of the few that is not impressed with Evo. Its shipping in the middle of an OS upgrade which it will not get right away (yes I know it will get it this year, Q4, but Q4 also inlcudes Dec. 31, 2010) and 4G is still in select areas. I believe the Galaxy S is shipping with 2.1 also but it will have Sony's OS layered on top like Sense. And like Sense it will always be late in getting OS updates. They're both HD capable so...
The only thing I can say is go by carrier. If you're on Sprint go Evo. If you're any other carrier, as I've stated in many other threads, go Galaxy (I hear all the major carriers are getting it, don't know when). In no shape, way, or form would I switch carriers to get a device that will sit atop the Android world for probably less than a month. Droid 2, Shadow, Etna, Venus, and Android's "God Phone" in December all mean the Evo's reign will be short lived.