I strongly suggest you get Prophecies.
You learn way more about the gameplay and the story from it.
Prophecies: A well made storyline (The flameseekers prophecies) and gameplay, take your time and play at your own rate through the missions and outposts, and the really wonderfull world of Tyria wich you can explore for hours. Change of landscapes, Burned land, Snowy mountains, deep jungels, seashore lands and deserts! I still play the origional GW game more then Factions just becouse of the world and story, so mutch more then Factions has to offer.

You have to experience it to know how good it really is!
Factions: far from a well made storyline (The Jade wind) that has noting to do with the Prophecies one to me (like Nightfall on the other hand will, GREAT!) Rushed missions and leveling (once on the main island you reach lvl 20 in a few hours...) Some great areas though (the Sing Jea Island, just couse of the landscape changes). The only bonus is that you get 2 new professions with it, But nothing beats the origional ones from Prophecies if you ask me. Factions is just about just that "Factions" and PVP.