Would have weighed in sooner, but I've been on vacation for the last few weeks...
My vote definitely goes to N. unicornis. Both of mine are right around the 12" mark, and are starting to develop the horn and bright blue caudal spines and trim (a good amount of yellow is starting to come in as well). They are very friendly and easy-going fish. My N. vlamingii on the other hand is much more reclusive, and can be a bit of a bully toward it's tank mates at times. Seems like I have the exception here.
I may have posted these pics elsewhere, but here are a couple recent shots of my N. unicornis...
...One more to show how thick these guys are.