I'd say the Galaxy S3...it's the best phone available now, or until next week when the new iPhone will be launched.
You might want to check out the mobile phone top 10 I've put together. It's got both of these phones and explains it a bit further. If you still can't make a decision I'm more than happy to give you some advice
I think HTC 1 X is best cell phone to buy but the software in HTC will be harder to you than the Samsung Galaxy SIII.