The Premium Edition is the retail release of Grotesque Tactics: Evil Heroes which was previously only available as a download and is basically an enhanced version of the original game. The new features are:
Voice acting throughout the game
Changing the armor of the character will change the actual appearance
The characters are free to roam the country in real-time when no enemies are visible
Improved graphics (replaced textures, completely new water)
Owners of the download-release can purchase an upgrade to the Premium Edition for a small fee.