Allow you to buy a backup body with your money
I guess I'm in the position I want a camera that will allow me to shoot cars and shoot fashion. For
me the D3 would be slightly better as I have all the advantages of that fantastic AF but with the dreamy IQ of a FF sensor, plus 9fps. If you want full frame with Canon, you lose speed.
I'm not going to spend £1000's of pounds jumping to Nikon when I can have a 1d MK3 for a steal, but the IV was meant to be the point Canon clearly took the pro lead. Instead they've hopefully played catchup (reviews depending) on a year old Nikon.