I find the debate between 5DIII Vs D800 pretty funny. Mostly because they're direct competitors and yet they're not. I agree, the have the same position in Canons and nikons lineup, but they're not really the same cameras. Funny thing is that the D800 are the true successor of the 5DII whare the 5DIII are the true successor of the D700, that is very general, but I think for most part it's true.
If you really struggle to decide which camera you want (economic consideration aside), then I'd say you don't know your needs well enough. If you do a lot of landscape, portrait and studio work or you really need to print big (Or if you're just a MP whore, which I guess most of us are to some extent), then go with the D800. If you want a faster AF, burst rate etc. you go with the 5DIII.
I chose the 5DIII, my combo was a 5DII and 7D. I hated the 7D, but it's speed was very welcome when the main body was a 5DII. The 5DIII really does bring the best from those two cameras together and the choice was clear to me. Between the 5DII and 5DIII I got my first kid and the speed suddenly meant a lot more to me than when I bought my 5DII.
So basically I bought the camera I needed, others are better off with a D800, but I actively chose and decided that not only ws the 5DIII the better camera for me, but it was also worth the extra money.
I don't really care if it will go over in history or it will be forgotten in 5 years, the photos I will get with it is what matters to me. Whatever I will be able to sell it for when it needs an upgrade, will show when that time comes, but so far the 5D series have held their resale value pretty well.