X-2. Square actually built upon the story and elements introduced in the previous installments and even delved into some of Spira's past in regards to the new plot unfolding. XIII-2's story may as well have been used for an entirely different game. It's like Square was so desperate to do a sequel they rushed to make up anything they could for a sub par game just to dole out DLC for people because they promised it for the first game and didn't deliver. The game was short and the story was just shoehorned into XIII's continuity.
X-2. Despite Vegnagun being completely stupid, I loved the battle system and enjoyed all the extra content more then what XIII-2 offered. Also lots of songs I liked too --- I'll eat a sock if VersusXIII is actually a mind blowing game. -PainsPerception