As someone who used to work in telcoms....this is a tough question. The Galaxy S 2 is good, but if you can go for an S3 for about the same price...DO IT. The battery life is much improved than the Galaxy 2, still boasts a beautiful AMOLED screen but it's BIGGER (how do you people with tiny phones SEE things?)
Also, quad core processor? Hellooooo?
Apple Lackeys - he's askin about Samsung VS Samsung! Going from Android technology to Apple technology is very difficult (much like you guys find going the other way difficult). Apple is more mainstreamed and you can't do much with it, Android boasts customisability which, if you're used to it, is difficult to do without. (Also, did anyone mention "back" and "options" buttons? Bliss!)
I've owned iPhones, Galaxy S, Galaxy S2, and now the Galaxy S3. Hands down, I would never go back to Apple.
My only advice - no matter which you choose if you want battery life do NOT get a 4G model!
Forgot to add - I had TWO Galaxy S2's that I dropped and broke within TWENTY FOUR HOURS of purchasing (still in contract for them too
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But my GalaxyS3 (which I dropped in the shop within minutes of purchase and at least once a week since) is still going strong. Phones with square corners are more susceptible to impact fractures (S3 is more rounded).