Well if you get galaxy nexus and i assume you going to buy it from google that way you staying off contract but if you going to get s3 you probably gonna have to sign up for 2 year commitment.. Unless u dropping 600 plus bucks. Both good phones i actually played with s3 today pretty fast device. I also have been faced with the choice between s3 and gnex. After comparing em I went with gnex. I have put a list of pros and cons for each device and gnex won. And i didnt really want to spend 600 bucks on the phone or sign up for another 2 year contract.
Pros: S3 got faster processor, better screen, better camera, lte, micro sd.
Cons: touch wiz, locked to carrier, no jelly bean, bloatware.
Gnex
Pros: jelly bean, clean interface, unlocked.
Cons: no lte(personally dont care about it) 5mp camera (dont care but i wish metering option was available like on s3 or s2)
Slower processor but fast enough for me not a gamer or heavy multitasking user.
no micro sd card slot(dont care 16gb is plenty for me) and its safer.
Good luck.. You can also buy European s3 which is unlocked comes with the quad core but no lte.