Cotton Boomerang is sort of an expanded edition of Cotton 2, with remixed levels and enemies.
The only real differences are in the gameplay mechanics, which aim to make the game more action-oriented by removing the health bar and the restrictions for weapon upgrading. Now that the health bar is gone, you get to select 3 different characters from a group of 8 that act as your "lifes" but each has distinct shooting patterns and abilities to enhance the gameplay. Additionally you can switch between characters at will (as long as they are alive) which triggers a bomb-like effect.
Also includes an art gallery featuring Cotton 2 and Cotton Boomerang material, additional difficulty modes and controller configuration options that allow you to map the command-shots.