personally i would stick with one that you turn on and off. Then you have control over what it does and when it does it. if you dont need to use the battery, then no reason to have it turn on. Also the external battery packs can take a while to charge. 4-5 hours depending on brand and how high the input is. I would personally wait to get a external if i was you. A lot of very good ones are right about to come out that are "apple certified". Then lenmar meridian and the mophie helium come to mind. They are a tad spendy, but you get what you pay for. Their output's are at 1000mah, so they will charge a phone like its plugged into a wall socket. Most of the others out right now are 400-700mah, which is more like being plugged into your pc. Also their inputs are higher so they charge decently fast. As of right now there are no really great battery cases out for the iphone 5. If you dont need a battery cases and dont mind carrying the cord then they're lots of options out there. My current external battery charger uses 2 AA batteries. I use 2500ishmah rechargable battery's, its something that will last forever. plus if im in a pickle i can always just grab some AA batteries.
Anywho, i wish you luck good sir.