If I were you- Id go to the store today and get Prophecies - OR if you have a debit card, simply download the client and purchase it through the online store- You could start playing TODAY and not even have to leave your house (for the next 6 months of your life

)
The thing is- Nightfall is going to be the most glorious game ever, and if you want to play guild wars, you will want to play nightfall.
Personally I really enjoy factions- however as a stand alone, it wont give you the epic feel that Guildwars truly is about. I mean, some features of Factions are totally awesome... but if you are a first timer it might give you a sour taste in your mouth.
reasons to get Factions -
1 playing assassin or ritualist. my ritualist was easily worth the purchase of the game. playing assassin in pvp is good too, but pve is impossible to get respect from other players. Mostly due to explode-on-death enemies that makes the assassin die a LOT, which gives most of their bad rap.
2 Alliance battles. People can say what they want about it, but its just plain FUN imo.
3 its not THAT bad - just not up to par with the original as far as storyline and the ability to truly explore the map in any way that you want... half the game takes place in the city - and the city is full of locked gates that you must earn the right to pass through...
4 easier to aquire max-level equipment and bonus attribute points
5 its geared toward pvp fans (even in the pve) which I personally dont mind at all. Im a balanced player.
Reasons to get Prophecies-
1 The searing is a great way to start guild wars (and I dont want to give too much away- but its chp1)
2 people are nicer, more helpful, more innocent.
3 the game simply takes more time to complete. if you play 4-6 hours a day youd be lucky to finish half of it before nightfall is out...
4 you have better opportunity to really LEARN your characters, and their skills.
5 prophecies gives you Skills as rewards for quests instead of just endless amounts of experience like in factions.
I still have a low level character in each continent and I see tons of fresh newbs (not noobs!) asking questions about simple functions of the game... so honestly, I would go so far as to say its not even too late to start your OWN guild even a couple months from now when you get the game down.
If you think you are more into pvp, get factions... otherwise get chapter 1 or chapter 3, then get factions when you get around to it- its not going anywhere. Factions was tailored to players that are experienced in the game, even tho it technically IS stand-alone