I'd get the S3 because it already has Android 4.0 and the S2 still has Android 2.3. Samsung takes a really long time to update their operating systems so you might not get 4.0 for a few months. It took them about 6 months to upgrade my old Samsung phone to the Android 2.2.
The S3 screen is huge too, 4.8" I think vs. a 4.5". I'd rather get the latest tech now, because in 2 years when you're ready for your next new phone, the s2 is going to look and feel ancient compared to the s3.