It sounds like you have decided to upgrade to FX, but which one? I would ask what you want it for? Image quality was my number one reason, with the ability to shoot in available light more often coming in a distant second. Image quality of course depends on much more than the camera body. Camera / shooting technique is paramount especially with full frame sensors (as I am learning... the hard way). But Lens choice is where it is really at isn't it? One can shoot all day but if the lens isn't up to par with the rest of the system, well, you can guess. Luckily I have some good glass, which is not to say I couldn't go for some more.
Now, with that said, I decided to go with the D600 and brother the IQ and what I can do in post processing due to the increased Dynamic Range is phenomenal! The reason I chose the D600 over the D800 was mainly overall cost. I would have loved the D800 for it's resolution but my existing pc's processor, ram, and storage would not have. As it is I did have to add a new storage array to my network and purchase some fast 32gb SD cards.
I guess what I'm saying is to consider the "extras" if you have a budget to work in. Good luck and have fun with whatever new toy you decide to go with.
---------------------------- A picture is worth a thousand words! I took a photograph and couldn't think of that many. I guess I'll keep trying!