Which Guild Wars to buy?

I'm going to start playing GW and am going to go buy the game today, I'm not sure which of the campaign to buy. Is there a difference with the gameplay or is it just the story. So, I need some different opinions on which to buy.

Thanks,
~ Jorox

EDIT: Nvm, just noticed the sticky about this lol. sorry for the post.

the first campaign is the largest and uncompareble
in factions there is factions wars
in nightfall there is heros
the top post on the firs page is on whichi chaper to buy

Sticky can be found at: http://www.guildwarsguru.com/forum/s...php?t=10102502