Galaxy S and Nexus S has similar processor/RAM/gpu. Differences are:
What they do not have:
Nexus S - No 720p video recording regardless of modding it, No FM Radio, No expandable SD card slot, No Gorilla Glass
Galaxy S - No Camera Flash, No Vanilla Android, No NFC Chip, No Three axis Gyro-sensor (Not a lot of games use this anyway), No Fast updates straight from Google
What they have:
Nexus S - Has NFC Chip, Three axis Gyro-sensor (Better for movement in 3d space compared to accelerometer in other smartphones), Fast updates and long life due to being a developer phone, Has Camera LED flash, Wolfson audio chip
Samsung S - Has 720p video recording, Has FM Radio, Has Gorilla Glass, Has expandable slot for upto 32 gb SD card
Nexus S vs Galaxy S:
Google Nexus S vs. Samsung Galaxy S
Samsung Google Nexus S I9023 vs. Samsung I9000 Galaxy S - GSMArena.com
Do not buy US phones if you care about warranty as it will be expensive to repair it. I would say Nexus S because the updates you get from Google is faster and no need to wait. Also Nexus S will get the ICS first while top dog dual core phones has to wait.
Go with LG Optimus 2X if you are into recording or playing 1080P videos, playing intensive 3d games. For general use, you will find Nexus S more than enough. In this case even Galaxy S is also good. Samsung Galaxy S GPS has also improved in current models compared to older ones released last year, in case some complain about that.