Agree with Geek. I had the Commuter for my Bionic and imagine it's a similar build for the S3. Biggest differences are:
-Commuter has a slick back with a silicone interior. Very slick actually, I didn't care for it.
-Defender is all rubber/silicone and has a built in screen protector which gives it some help with moisture/water.
-Commuter has a slimmer profile, but not much more than the Defender.
-Both have covers for all ports to defend against dust/water.
If you tend to drop your phone quite a bit, I'd highly recommend the Defender. I know a lot of folks who have one (mainly those with iPhones and/or kids).
Here's a massive thread to go through if you want some with actual cases for advice:
GS3 Case discussion thread