well i was going to tell you to wait til GWN comes out, simply because itll have a fresh skillset, 2 additional classes, etc
Factions is a good game, and I think its very very fun once you get to some of the later missions... but for a new player- it might gimp you having a lvl 20 character with hardly any real experience...
Prophecies is a great game. You wont regret getting that game first.
Id have to recommend this order: GW1, GW3, GW2
reports say that gw3(nightfall) will be more focused on pve, which the community here assumes will mean a slower levelling progression to maximize the feeling of growth and accomplishement.
Personally id prefer something about halfway in between factions and prophecies... and I have a feeling Nightfall will be it.
Yeah dude- Factions may be hot right now- but as soon as gwN comes out its going to slow down (currently its a freaking madhouse til you get to the 'split' between the jade sea and echovald forest) so dont feel rushed into getting it quite yet. Take your time and enjoy prophecies (it is THE original lol), then if you like it, get Nightfall (or wait to see reviews).... THEN get factions last