I depends how heavily invested you are in Canon gear and if the lenses you need are available for Nikon. Myself I would've loved to get a D600 over the 5D2 I just got but it would have been too expensive to switch. The primary reason why I would've chosen the D600 is the increased dynamic range.
I would also miss my 200mm 2.8 since Nikon doesn't have one of eqvivalent quality and price. I use that lens a lot for portraits.
That is more like huge increase in DR (over 2 stop). Dont really care about the AF as I usually use manual anyway. Build quality will be better, well it is weather sealed at least anyway.
Yea having played around with quite a few raw files, the D600 has way better dynamic range.
). The 2 stops DR advantage is only @ ISO100, @ ISO400 and up there is no real advantage anymore.



