Biggest thing that you'll want to be concerned with is the mAh. Obviously, the bigger the number, the more power you'll have and longer battery life. The batteries come in either case form factor or as a separate battery which you connect to your phone with a cable.
The larger battery cases will make your phone thicker and the battery cases with smaller mAh will be slimmer. Not sure if this is an issue for you or not. If you want to keep your phone as slim as possible, then you might want to look at the battery packs and just use as you need it.
A lot of people I've seen with these usually get the slim battery cases. While they don't give you a lot of battery power compared to other larger cases, it'll give you enough extra juice without bulking up the phone too much.
The type of case you want is really up to your personal preference. If you need lots of juice, get a battery pack or larger battery case. If you want a little extra juice without too much bulk and not have to carry around a battery pack then the slimmer battery cases would be your best bet.