Nickle coated casings wont tarnish like brass and they load and eject easier (less friction) than a brass casing. As for shelf life, if stored normally, you will see no difference. If they are constantly getting wet or kept in a cartridge belt that can retain moisture (leather), you will see the difference. You will not get as many reloads as they are made of a harder material.
Addtionally, they are easier to see if you are doing a "1/4" chamber check" (pulling the slide back just enough to ensure the gun is loaded) in a low light condition.