There is nothing wrong with Nikon's Pro level Cams. One of the biggest differences between them is that Canon has a higher megp then Nikon. However that's not a bad thing. It all depends on what style you are shooting, at a pro level, that determines what kind of camera you need.
Frankly, in my opinion, you don't really need much higher then 12megp. If you are trying to shoot higher then that cause you want to make a very large print, or something along those lines (especially when it comes to portraiture and studio work) then switch to Medium Format.