I have used the D600 for the last couple of months, and I've managed to cover everything I need with it, but a new job assignment now means that I have to cover a lot of sport as well. I don't find the 5.5 frames per second fast enough, so I'm asking you guys to suggest some alternatives for a backup camera. I would be willing to spend up to $1700-1800, and the most important things are of course burst rate, autofocus and ISO capabilities. I will be shooting outdoors and indoors, and my lens selection contains just FX-lenses.
I have been looking at Ebay for a used D3, as well as D300S and D700 - both with battery grips.