I have the Galaxy S 3 too. I like to use a GPS program called alpinereplay. It tracks my speed and where I have boarded. Anyways... instead of getting a backup battery, I got an external backup battery. The brand I got is Anker. It is a very nice product and they ship really quick. Ordered Tue and had item by Thurs. The first one I purchased was the 3200. It will take the battery from completely dead to 98%. I liked the product so much I purchased a bigger one the 8400. This one can charge the phone completly 3 times. It may be longer after I brake in the battery. Oh anther thing I like about this external battery is that it has a USB port to charge anything.
I have not used an external battery but I dont think it would be that bad since the Galaxy S 3 is so thin. I put a bumper on my S3 just to make it a little thicker.