I'm going to give my opinion as a new player that bought Prophecies about a month ago and played GW:EN preview liked it and picked up the full version.
If you're looking to get in and decide on getting Prophecies i might suggest picking up the Platinum Edition and get GW:EN bundled with it. While many players seem to have a very mixed opinion about GW:EN and i DO understand their arguements, as a new player GW:EN is full of interesting new content if you're looking for some challenge or variety once you hit level 20.
I'll give a couple of the main arguments against GW:EN and why you a perspective new player won't care.
1) "Reskinned endgame armor": Ok so the GW:EN armor does look as "new" to an old player, it's still new to you as a new player so it doesn't matter. It's all going to look pretty cool.
2) Hall of Monuments: Alot of older players are fussing about it because it doesn't cater to their vision of how they want their titles displayed. Meh, you're new we're still getting started and titles mean little to you as a new player. If anything the HoM serves as a motivating factor for you as you'll want to fill it's empty displays with your accomplishments and each new statue you add will be a thrill of accomplishment as it starts to fill. I got a nice feeling of contentment when i managed to finish the Prophecies Campaign and display my first statue in the HoM, now to work on Cartographer and Skill hunter and try to add a couple more.
Now for why I liked GW:EN as a new player and you probably will too.
1) Loot: The loot in GW:EN blows away much of the loot in the Prophecies campaign until you get to the very end. You can Access GW:EN once you hit level 20 which is fairly easy to do, you'll probably reach it about half way through the campaign. At that point in the game Items you can pick up in GW:EN are VASTLY superior to what you will be getting in Prophecies. Also it gives you access to inscribable weapons you normally wouldn't have access to without owning Nightfall.
2) Heroes: You have been leaning toward Prophecies for whichever reason and that means no heroes for you normally as they are in Nightfall. However Buying the Platinum Edition and having access to the GW:EN content means you get the GW:EN Heroes. If you like to play solo or with only a couple close friends Heroes are a HUGE help as they are vastly superior to henchmen.
3) Challenge and variety: Access to the GW:EN content means you get to fight much tougher enemies than you would in Prophecies until later in the game. Also the variety of monsters in GW:EN a nice addition, and since Prophecies will be your "home" campaign you'll probably get a decent hate/interest for Charr and be thrilled to be able to go stomping through their homeland and learn a bit about them.
Just a final parting warning if you do buy the Platinum Edition, your account key probably will have a 14 day/10 hour trial key for the other campaigns as part of it, so when you make your charactor make SURE you make it in Prophecies, unless you understand beforehand that the charactor in the other campaign is temporary and is going to be locked out after the trial period is over. I wouldn't want you to get upset at Guild Wars because you spent your 10 hours pouring your love and time into a Factions or Nightfall charactor and then be surprised when after 10 hours they go "poof, locked until you by this campaign."
Good hunting and i hope this helps any perspective new players that are wondering about Prophecies vs the Platinum Edition that is Prophecies + GW:EN make a more educated choice.
Good Hunting, and see you in game