Tower Defense and Kuh, Boom are mutually exclusive achievements available in Saints Row: The Third. Both achievements are worth 20 GamerScore each.
Both achievements are available at the end of the Act 1 finale, ‘The Belgian Problem’. Which achievement you get depends on the choices you make while completing the quest. In order to get both, make sure you save your game before you start the quest.
To get Tower Defense, you must choose to defuse the bomb. This will also reward you with a permanent 10% cash bonus, and you will receive the tower as a crib. This is generally seen as the preferred approach.
To get Kuh, Boom., you must choose to detonate the bomb, destroying the building in the process. This approach will net you a permanent 10% respect bonus.
It’s worth noting that your choice will affect how the remainder of the game plays out, in terms of the story.
Here is a video showing how to complete the quest.
Thanks to AchievementPimps for the video!