Upgrade D3 to D4 or D4s?
I went from the D3 to a D4s. The AF is better with the D4s, and the frame rate rocks. However, if I could do it all again, I would seriously consider a used D4. My D4s has been in for a shutter malfunction 3 times in five weeks, and the D4 loaner I am using is itself a wonderful upgrade to the D3.