I've done a little video with my D800 and the quality seems fine, even the sound from the built-in mic when I didn't have my external with me. I've used it up to ISO 1600-3200 and I was quite please with the clean 1080p from it.
Though the Sony has a lot going for it, if you are in the Nikon system and have the glass, I'd stick with the system and not reinvent the wheel as far as learning a new system.