I have all three 'episodes' of Guild wars. I can't really recommend one more than the other - they are both very good. If you like the idea of being able to heros in your party (including...getting them, level them up, alter their skill sets, how they attack. and more) then Nightfall is for you. You can of course take your prophesies characters to the Nightfall area and they will be able to use heros (after one or two nightfall quests). I have not got very far in Nightfall or Factions since I am very much involved in the prophesies campaign still. So I can not compare how they compare further into each campaign.
Someone might have a valid trial key for either of them which would enable you to try them for a week. I'm not sure if these keys are still around tho - or valid. I think they came with the collectors editions of the game.
Good luck
