i gleamed this from a faq on
www.guildwars.com
Is Guild Wars Factions an expansion pack or a stand-alone game?
Guild Wars Factions is a stand-alone product. Those who own Guild Wars are not required to purchase Guild Wars Factions in order to continue playing on their game account, and those who purchase Guild Wars Factions may play without purchasing Guild Wars. However, there is a strong level of interconnection between the two games that makes having them both an exceptional experience.
So..
you can choose which ever one you wanna play no need to drop 100 bones