Having had both the S3 and an iPhone 5...
As a standalone phone, the S3 is a little better.
As someone with an iMac, Apple TV and iPad, the iPhone fits my life better and the benefit of easily shared content outweighs the slight downgrade in screen-size and open source-ness. It is the first iPhone I can say that about, I've been BB then android prior to getting this. Echoing sentiment above, the camera is amaze-balls too!
Sooooooo - the choice be yours.