The Third Rail case differentiates itself by offering a removable battery that attaches to the back of the cases and can be daisy-chained (you can connect up to six batteries).
The nice thing about the system is that if you move on to the iPhone 5, you can keep the detachable battery and use it with a new case (Third Rail says it's sticking with the same system for future battery cases). The one downside is that the batteries (1,250 mAH) aren't as high-capacity as those of some competitors. Also, the case and battery combo is fairly pricey at $90, and extra batteries will cost around $60.
But it's a great concept that allows you to keep your iPhone in a slimmer case and only attach the battery when you need to.