As did the first two responders, I have both, and they are both excellent in their own way. It is well worth owning the pair together.
But if you had to go with just one for now, it looks like you are coming from a D700 with a lot of Nikon gear - so you are certainly no newcomer to the Nikon product line. Based on that, I would start with Thom's. He goes through a lot of the science and technology that is in the D800, with just a touch of historical perspective of how these things evolved in the Nikon product line that may be of interest to you. It's also 850 pages long, but only available as a PDF - he said he has no intention of printing it.
If you want a printed book, which I happen to like better than PDF's, get Darrell's book first. You won't be dissappointed.