I would go with, amongst the many good choices actually, the D600.
I do a ton of low-light when we travel, and many of my rewarding photographs are night and or dusk shots. So I think any of the FX cameras are preferred as a travel camera and they all have very good low light performance. The D600 combines low light and smaller form factor, both good things in a travel camera.
The D5200 also intrigues me and really there are a lot of great choices aren't there. I guess DX has the possibility of some smaller DX lenses as compared to FX.
Having said all this, since I have a D800E in my kit that is going to be my travel cam, and I won't purchase a D600. I also have used my D4 as a travel cam with my 24-70 on a Black Rapid strap and it works But before our next major trip, I think I will get a 24-120 f/4.