Michael, My recommendation is to rent a D600 for a weekend and put it though its paces. If you are satisfied with the images you capture, than purchase the D600. If you are consistently missing your ideal shot and believe it is because of the slower FPS of the D600, then go for a D3/D3S. List your pros and cons of D600 vs. D3 and see which body offers you more. Only you can decide.
Hope this helps...JL
________________________________________ 45+ years of Nikon ownership and counting Visit my SmugMug gallery