There are two achievements that depend on a choice at the end of the single player campaign:
-Judge, Jury and Executioner
-Man of Conviction
In order to get both, choose one decision, then, immediately after the achievement pops, hit the start button, and choose "Retry Last Checkpoint". Play through the end sequence again until you get to the choice, and choose the other option.
You can get both, but you have to be fast in order to not miss the end checkpoint and have to replay the entire chapter.
The only choice that matters is the very end one, where you decide to shoot, or to spare.