The chimney cover is protection for the entire chimney and chase. While a chimney cap only covers the flue(the area where the smoke comes out) these covers span the entire crown and affords protection to the entire system from the top. The crown is a very important part of your system and needs all the protection you can give it.
Black Galvanized is the most affordable you would think, however they are susceptible to rust over time. You can remove it and repaint it from time to time to keep it in good shape or you can get a stainless steel model which carries a lifetime warranty.