I didn't have the D70s but I was moving from D7000 and I can tell you that D600's Dynamic Range, ISO, and IQ are very good. If you have the money to move up and to buy a new FX wide angle lens for your landscape work, then I will say go ahead and upgrade.
4 reasons I made the move: 1. Need FX for high ISO (I shoot a lot handheld in low light conditions) 2. D600 has the similar buttons layout as my D7000, nothing too new for me except sensor. 3. D600 fits me perfectly (weight and size) 4. Price.
Remember, suggestions and ideas from others are good but always buy something that fits you.