Topside is much easier to shoot beautiful video with most cameras, but full frame is much more difficult to shoot underwater. I find the Nikon cameras have more noise compared to the Canons (with the exception of the D4, which has other video issues).
Depending on what he shoots, the D600 is a fine combo camera, but for video noise, it loses out to the Canon 6D. For video, the Canons have a bigger developmental base such as color profiles
It really depends on what he wants to shoot and how much he wants to spend.