A lot of variables go into this decision. Some of your subjects lend themselves to slow and careful work, and you should be able to reap the benefits of the D-800. But some things, like walk-around photography or hiking out into the middle of nowhere for a great landscape, might be better suited to a lighter camera.
What type images do you intend to produce? If you are only posting small jpg's to the internet you probably don't need either camera. But if you are printing big the D-600 would be a good choice. And if you are printing REALLY BIG, the D-800 would make sense.
Don't forget to factor economics, both the initial purchase and how an accident like dropping the camera off a cliff in the field would affect you.
Ultimately, you will have to figure that out for yourself. For me, at this point in my life, I am going with the D-600. I waited until the specs were confirmed, and if there were any surprises I would have bought the D-800. They both look like exceptional tools.
There are at least another dozen things to factor, but you have to decide what is most important to you and base your decision off of that.