Since as you've said this going to FX is going to happen, then it's just a matter of which FX as I think you mentioned!
Though I have thoughts about Nikon's I don't have, I can only really say on Nikon's I do have or have tried, which is the D700 and D800E.
Since I don't believe the D700 is of interest to you,I'll just say my 2 cents worth about the D800E that has been my dream Nikon from the beginning!
The D800E Nikon is a specialty camera, and some might find the file size bigger than they are worth! The 24 MPs Nikon seem to be the preferred by general shooters! So in some cases this might be true, but mostly I think sports and birding would be the biggest areas that other Nikon's either could or would out shine it in! But in macro, regular wildlife,street, and especially landscapes this awesome Nikon holds it's own or excels! Now don't get me wrong, the spray and pray method, or more controlled versions like that will put this model in the less likely to please list! But for most other methods this is an outstanding performer!
Now to the dreaded file size issue! It is real and even if this Nikon did handle the spray and pray method, nobody would want to load their computer with that amount of these huge files! Lol I can't even imagine that outside of a nightmare! Lol
But on the other hand here I am with less ability to handle these files than most, and I am still doing it though on a very limited amount at a session or on a card! What it would take to shoot full bore and post processing a big card full of these files, would have to be asked of somebody who has done so! Which by the way is what I'll be looking for in my next computer! Lol
As to if this Nikon would the right one for you, only you can answer that one buddy! But I will say I love mine and it is worth every penny of what it costs IMHO!