I agree with you that ergonomics is a very important factor. I rented a D600 for a week to evaluate how it felt in the hand as well its image quality. I have a D7000, so my comments are relative to that camera.
You are the only person who can really say how the grip feels in your hand. It is only slightly larger than the D7000 but moulded in a slightly different (more comfortable for me) way.
At first I thought the multi selector was very small but after using it for a few days I thought the D7000 multi selector was very large. For me, it's something I got used to immediately.
Other comments on ergonomics are:
-I prefer the larger viewfinder of the D600 -Eye relief seems the same as D7000 and okay for me wearing glasses -The sound of the shutter firing seems about the same as the D7000, possibly a bit quieter in Q mode. -LCD screen seems slightly better and it is a tiny bit larger
If you consider the User Interface an ergonomics issue, the D600 is a very easy move for owners of the D7000 with a nearly identical layout of menus and buttons.
It may be a larger change for someone accustomed to a different control layout or wanting to use two bodies with different layouts.