The differences is if you play online you usually play against other humans which sometimes can make the game more challenging but also more fun. If you play offline then you are playing against the computer.
The main difference all boils down to whether you wanna play story or arcade mode type of games or whether you like the social interaction type game sin which there are other online competing or player.
You can have a blast either way playing with them online or just sticking to clearing your own campaign games or arcade games.
