Both the Samsung Galaxy S II and the new model run Android 4.0 Ice Cream Sandwich with TouchWiz on top. And while the general interface is quite similar, the one on the Galaxy S III is more polished and it includes even more goodies.
The lockscreen for one is much more functional - four shortcuts, weather forecast, dual-clock when roaming, news ticker and nature inspired aesthetics are appreciated.
ICS brought face unlock to the S II, but the Galaxy S III takes things a step further - Smart Stay uses the front-mounted camera to make sure the phone never locks or even dims its screen while you're in the middle something. The whole face detection thing also got into the gallery with Buddy photo share and Social tag able to recognize people's faces and allow easier to access their contact and SNS info.
Messaging on the S III also got improved with some clever features - direct call allows you to put the phone up to your ear and dial the contact whose info or text message you were just looking at. Smart alert will have the phone vibrate when you pick it up to notify of missed events - that's great for the times you just grab the phone without even looking at the notification LED.
There are a few other nice tricks (double tap the top of the phone to scroll to the beginning of some lists, Pop up play and so on), but the biggest potential game-changer that the latest Galaxy S brought was S Voice.
We've covered S Voice in a blog post and in the S III's review, so you know it needs more work done, but it still doesn't seem ready to enter your daily routine. You still might prefer it over Apple's Siri though - particularly if you're a Spanish, Italian or a Korean speaker.
Samsung's Galaxy phones typically come with an AllShare app that serves DLNA duties. The Galaxy S III, however, adds AllShare Cast support, which uses a wireless dongle to stream HD video.
The Galaxy S III also brings MusicSquare along with updated sound-enhancing technologies. MusicSquare is similar to Sony Ericsson's SensMe and builds playlists automatically based on the desired mood of the songs.
The Galaxy S II was one of the first devices to support Wi-Fi Direct - a technology that combines the easy pairing of Bluetooth with the high speeds of Wi-Fi. The S III builds on that with S Beam - the data is still channeled over Wi-Fi Direct but the pairing procedure has become as simple as humanly possible over NFC.
The biggest weakness of S Beam is that currently only the Galaxy S III supports it, but Samsung is sure to put it on more phones. As for inter-maker compatibility, the Galaxy S III can still do the simple-but-slow NFC transfer or the fast-but-slightly-more-complicated Wi-Fi Direct transfer.
Both the S II and S III can do fast Bluetooth 3.0 transfers too, but the S III tacks on Bluetooth 4.0 LE for accessories with low-power, low-bandwidth needs (think heart rate monitors, smart watches, etc.).
Knowing the Galaxy S phones are surrounded by an active hacker community, a lot of these software tricks have a very good chance of landing on the old Galaxy S II.