In reviving this topic, I'm contemplating the D600 or D3s.
Currently a D200 shooter and get paid to shoot rodeo sports. I can get about 85% of the images I need from the 6FPS that the D200 gets, but will miss shots at times for things like barrel racing, reining horse and roping events. Hence, the consideration of the D3s.
However, I also shoot these sports using single shot and strobes (covered arena), so timing is more important than frame rate.
Thus, I think I can work with either for what I do there.
Where I am on the fence is for low-light stuff. I'm looking to market myself as a second-shooter for events, weddings, concerts, etc., so low light and image quality are important.
I know that 24MP doesn't necessarily mean better IQ, but you do have more pixels to work with if you have to crop in. Also, I know the D3s ISO goes to 12800, but you don't judge how "fast" a car is by the numbers on the speedometer.
Therefore, I'm trying to figure which of the two would be better for lower light stuff AND have better Image Quality and those ISOs. The D600 is newer and likely benefits from advances in technology. However, the D3s is the "king of the hill" (or was) in low light performance.
Given the D3s is nearly twice the price, used, as a D600, that is another factor under consideration.