The higher the tier the bigger the reward. Most of the tier contains the rewards listed in the previous tier.
*Example: Recruit tier contains Digital Wallpaper. Officer tier contains the Digital Wallpaper and everything listed within that tier.
However, some rewards do not stack.
Example: Officer tier rewards 1 Digital Download of an Artbook of your *pick. The Agent tier rewards you with 2 Digital Downloads. The reward will not stack, you will get 2 books not 3.