It's both. Think of Magic the Gathering. You have the old set, and you have the new set. You can play with either set, or you can play with both sets. And the boxes all cost the same.
The campaign's different. It's on a different continent that's more Asian-themed. It has two new classes, the Ritualist and the Assassin. The old classes are there as well, but they get different skills than the ones found in the original game.
If you have the original game, you can go to the new continent through a quest in Lion's Arch. Your character must have ascended already.
If you only have Factions, then you create a new character that starts off at an island close to the main continent.
Some other differences are: 12 vs 12, alliance battles that actually affect territory control between the two factions. The quests give a LOT more exp, and the missions as well. There are three levels of rewards in missions, so if you do really well, you can get all the rewards, which is around 4-5k exp and 3 skill points.
Some of the missions are also more detailed, for example, one mission has two entry points. One for characters from the original game, one for characters who began in the Factions continent. So you have two teams trying to complete the mission at the same time.
Of course, there are new henchies as well. For example, one of the henchmen is a Tengu fighter.