Should I get Guild Wars Faction or Nightfall?

I am nearly halfway through the original Guild Wars, Prophecies, and looking for more! I can’t decide between Nightfall of Factions.

I want to be an assassin AND a ritualist, and i have NO interest in paragon and dervish. Well, i hear dervish is fun, so I might try it out.

I will probably end up buying both eventually, but I want to know which to buy first. I cant buy both right now.

Thanks! :)