Definitely go with an Anker one, they're cheap, And have great amounts of mah. Also the juice packs are very bulky. They do only add a little to the bottom of the phone, but on the 4S, the juice pack is 17mm thick. The iPhone 4S is 9.3mm. So it will just about double the phones thickness.
IMO I would go with this one, half the price of most Mophie cases and ten times the power. I have the smaller lipstick size Anker for daily back up charging and it works great.