I find DA to be a more impressive accomplishment from a design perspective, in that they managed to create a tremendous amount of backstory and setting detail without the crutch of having decades of Forgotten Realms materials from which to take things.
DA also has slightly more replay value for me because of the differing origin stories, and because having only three classes allows for three fairly distinct play styles.
DA's tactics system makes it fairly simple to allow most of your party to function on autopilot while you focus on key aspects of combat. This really only makes a huge difference with spellcasters, which had to be micromanaged a bit more in NWN2. Speaking of spellcasting, the quick regen of health and mana/stamina outside of combat is another plus, as it eliminates recovery waits.
On the other hand, NWN2 was easier to understand out of the box because of my D&D knowledge. I'm still trying to figure out questions in DA like, "What monsters do spirit damage that would make the spirit salve worthwhile?" I had to lookup a forum post to figure out how theivery skills worked, etc.
I also thought NWN2 had a more epic story and more humor, and is somewhat lighter in tone than DA. Sure, bad things happen to people, including members of your party, but nothing really comes straight from the Book of Vile Darkness. That makes it fairly entertaining to me. Whether or not this is better is simply personal preference. Obviously, any big fan of the Forgotten Realms as a setting is going to appreciate the game's locale.
I would say DA is the better game, but DA also came out 3 years later than NWN2 and got to take advantage of feedback given in response to NWN2. I found NWN2 to be a vast improvement in many ways over the original NWN.