I decided to get the D7100. I haven't used it much for spotting yet, but the ability to focus at night on aircraft flying in the distance using a 300mm (F5.6) lense seriously impressed me! I'm sure the D600 would be just as good if not better. My reasons for the D7100 (upgrading from D80):
1) DX format - You can never have enough zoom!
2) Low light performance - I reckon I'll be able to use ISOs up to 2000+ and not have noise issues
3) Frame rate - 6fps (although the buffer isn't that big. Just ordered an Extreme Pro card which will help)
4) Weather sealing - No longer have to stress about putting it away the second there's a spot of rain.
5) Cost - cheaper than the D600
6) Dynamic range - should be 2 stops better than the D80
7) CLS commander mode - Not available on lower DX cameras
I never really considered the D600 because of the FX
format and increased cost, but I'm sure it'll be an awesome camera!
If you use it for more than just aviation then FX
is probably the way to go.