What needs to be considered when buying a phone case?

A case that covers the whole phone or only the back of it?

The case that covers the whole phone usually has more features and it also protects the screen. However, using the phone and answering calls requires opening the case first. The case which covers only the back, leaves the phone usable at all times and often maintains the slim profile. The screen, however, will be left unprotected, although the edges in most cases come a little bit above the screen, giving protection when it's on the table screen facing down.

With cards slots of without?

It's worth considering whether you want to use the phone case as a wallet as well. Often there is enough card slots for normal daily use, even so that you can leave the actual wallet at home. The case which covers the whole phone usually has lot of slots, working as a phone wallet. The cases protecting only the back, usually have room for just few cards. You can also keep the wireless card on top, so you don't have to take it out each time.