I have a GS2 base model. I'm sure your aware of the GS2 sky rocket and now the S3. Really for me it's all about the speed of the device and how fluid it is during usage. I've used a LTE phone and it's quite a difference when your on the road off of wifi. When i first read about the GS2 last year and wasn't coming to the US for months i knew from the get go it was phone to get. I wasn't trying to jump on the bandwagon and all of the reviews confirmed what i saw.
2011 the GS2 was voted phone of the year and had rave reviews for what it accomplished vs other android phones. Pretty ground breaking vs what i saw other people had and played. The GS3 is a great phone. Some of things i notice off the bat - picture taking has no lag, quicker processor, more ram, bigger screen and support for LTE. These are the things that will make the user experience more fluid.
I have played with an Iphone and i can see why people love it - user friendly, easy to use, and lots of support for it. Rock solid performance. It never really needs rebooting. My android avg about once a week of rebooting.

Go GS3 for sure, you'll love the bigger screen 4.8" and no delay picture taking and simultaneous vid / picture taking. The extra ram should help keep it speedy as well.