According to an interview with Adam Tierney, the game's director:
We had a tremendous amount of fun retooling the script in each of the different character’s voices, and the end result ensures players will always read something new dialogue, and get a slightly different perspective on the game, depending on who they’re controlling. We even tweaked the ending a bit for a few of the characters (shhhh).
Which characters, or sets of characters, have different endings? Does anything else affect the ending, such as keeping all your marines alive or completing the game under a particular time?