Think about this, you can get the S3 now and be locked into a contract and a phone that may or may not have a decent operating system. If you like the iPhone 5 once it's released, how are you going to feel about the contract you're now locked into with the S3? You can always buy out of your S3 contract later but it would seem a waste of money if you decide the iPhone 5 better suits you.
Also, one of the reasons I dumped android was that the OS has undergone 3 major changes in as many releases (froyo to ice cream sandwich). Is Google going to change the UI layout in the next version like they have done in the last three, forcing you to relearn the UI?