Get factions. You dont need any of the other 3 to play it and i think thats more challenging to play, i have all 4 (prophecies, factions, nightfall, eotn) and personally i favor factions. It also levels alot faster so you can stop worrying that early on. It does tend to get harder faster though, if you don't know what you're doing, but there are always people to help you. Definately get this, since it's entirely different from WoW or any other mmorpg i know. Prices have dropped alot, so theres a bonus. Who knows, you may come to like it and buy the others too. Guild wars 2 will go more in a general mmorpg direction though (leveling and stuff). Guild wars 2 still has a long way to go before its release, so i wouldnt bother with it for the moment. By then im sure you can grab another 50 bucks.
Also, darksoulpriest, you suck. This isnt the only topic where you act as the wise guy and only critique people that atleast TRIED to give a decent respond. If you don't like it, give a better answer yourself instead of bashing others. And your analoge did stink.