What will it offer
Best to follow the Apple website, or another of the enthusiast blogs to learn what's in the latest release.
Word on the street is that iOS7 will be released simultaneously, so there maybe many, many changes from the iPhone 5. Likely incremental changes in hardware.
That's probably the issue is that you have a 22 mth contract left. You will have to do a renewal, and assuming you are not on CLP, you will have to buy out the rest of your current iPhone. Good news is that you can still sell you iPhone for a little bit of money to cover the cost of getting a new iPhone but it probably won't be worth it in the end. On top of that, you are running the iPhone 4S. You honestly are not missing out on much when it comes to upgrading to the iPhone 5S. Slightly bigger screen, faster processor, etc, but most of the upgrade comes from iOS7 which you can download and use.