When you had your D300 and 17-55, did you shoot most of your photos at 35mm? If so, go with D600 + 50mm. If you shot at 55mm a lot and still felt not enough long, go with D7K + 24-70.
1 year or two later you will have both, FX and 24-70, so take it easy. Even if you make a "wrong" decision it's going to be fixed later anyway.
I have D7000 with 24-70 and I love this combo, except that the 24-70 is too heavy and the autofocus of D7000 is kind of weak. Anyway I prefer this combo over D600+50mm prime. I use 24mm a lot and 70mm too. I know it's a FX lens and I'm "wasting" it, but I too don't really need 18mm that much and 55mm is not enough long for me, so I'm enjoying it.
It's interesting that people saying using a FX lens on a DX body is a waste. Using a prime on a FX is not a waste? A FX camera can do 24mm, 35mm, 50mm, 70mm, with a 24-70, limiting it to 50mm is not a waste?
Also, I see people saying a 50mm 1.8 is sharper than 24-70, or IQ of D600 is much better than D7K. Well, when you shoot brick wall, yes, it is. In the real life, a decent zoom lens is much more useful that a prime, we all know that.