PHXAZCRAIG wrote:
Having owned an 80-200 since 1994 (and currently using a 70-200F2.8), I know exactly what you mean about the 70-200 being too tight on DX. I finally just stopped carrying mine altogether (in favor of an 80-400AF-D).
Once I moved from DX to FX (D700), that 80-200 became wonderful again. In that respect, you'll love the D600. It pairs much better with a 70-200 than any DX camera.
Aside from specific lens 'fits' to FX, you'll see that the D600 opens up more options for shooting in lower light. That's about the only reason to move from DX to FX, unless you are also chasing shallow depth of field. You will of course have to realign your lenses to the camera.
Having shot a n8008s/n90s mix, then a D200/D300 mix for years, it was interesting moving to a DX/FX mix (D300/D700). Because of the lens alignments, it really was not a good mix for me. I like to shoot with one body over each shoulder, with complementary lenses. (24-70 / 80-200, or 12-24 / 24-70). That mix rarely works with FX/DX, though it might in your case, with 17-55 on DX and 70-200 on FX.
Once I got my D700, I only used the D300 for wildlife. Once I got a D800e, I don't use the D300 for anything, and I'm trying to justify keeping it. I barely use the D700 as well, since the D800e does so well. You might or might not end up the same, with one camera (probably D600) getting the bulk of the use, and the other serving some sort of specialized function.