I own both and I wouldn't say one is better than the other, just different - and different enough that you would prefer one over the other, but similar enough that you could play either just fine.
Wilson quality control notwithstanding, I find the K90 a little easier to swing with a more 'lively' string bed that provides easier access to spin and power. My serves, backhand and volleys are all a little easier and more effective with the k90, though my forehand ground strokes seem a little more consistent with the n90, all things (strings) being equal. I've upped the tension in the k about 5 lbs for optimal results.