I have a Rode K2, which i love, and a friend has an NTK. We are working with 2 singers, one of each gender. We knew the NTK sounded quite good on both voices, so when i got the K2 we did an A-B test with both voices. For the female voice the K2 was a clear winner. Much warmer, more intimate, and better defintion. The male voice sounded nicer on the K2 as well, but not so vast a difference.
On the other hand i play guitar, and i was a bit disappointed with the K2 on that. I just couldn't seem to find a good position for it. It just always seemed muddy. I borrowed the NTK, and seemed to get a better sound immediately. I really like the sound of it on my guitar, albeit in some rather odd positions. I've been recording quite a bit of guitar, so we've swapped mics for the last few months. Once i finish this project, we'll swap back, and i'll see if i can get a better sound out of the K2.
In summary, from our experience, the NTK is a better all-rounder, while the K2 is a much nicer vocal mic. The K2 has a control that gives infinite variation between omni, figure-8 and cardiod patterns, which is nice.
Hope this helps... N.