Hard question to answer because they are both very different.
Guild Wars has tons of content, is updated frequently (well, it was when I played), and has a strong PvP scene if you get bored of the campaign and side quests. On the downside, finding a decent party can be difficult - plenty of idiots to spare and some missions you can't do with NPCs only, you can't pause the game (obviously) which can prove troublesome when you are locked into a 3 hour mission - which is often the reason you'll have people dropping out of the party mid-quest or just sitting there AFK waiting to reap the benefit.
Neverwinter Nights 2 has a very strong single player campaign, much more engaging, plenty of paths to take depending on how you feel and obviously you can pause/save at any time so it's easier to "pick up and play". The potential for creating your very own character is absolutely HUGE too, so many classes and the option of multiclassing & prestige classes. Guild Wars has plenty of options for characters too in the form of skills that you take along, but I think NWN2 requires more planning if you don't take the recommended options and the result can be very rewarding. NWN2 also has an excellent online community, with people writing modules for it all the time including "persistent worlds" which emulate features of MMORPGs. So basically, you are always going to have extra content to play with. The quality however depends on the people writing it. It's also more of a resource hog. It plays fine on my Barton with 1GB RAM & 6800 Ultra albeit with medium detail, but still looks better than NWN1. The load times do bug me a bit though.
I think my personal preference would lie with Neverwinter Nights 2.
