I was also using a 300s and I am now using an 800 as well. I have not had the 800 long, but have used it on a couple of shoots and I find it very comfortable switching back and forth between the cameras on the shoot. I fumbled around a lot more between my 300s and D80 when using two bodies on a shoot. They did change the control for the focusing a bit in the 800, no longer on the back, but an integraged AF-M switch with a center button that activates the Sub Command dial and Main Command dial for these changes. I have not seen or held a D600 in person, so I cannot comment on the difference of the size or the controls.