what are your goals photographically? what will a FF dSLR do for you that your M4, M8, and X1 don't do now? if you don't know or your mindset is more semi-idle curiosity about what dSLRs offer, then any fullframe dSLR will give you a taste. why don't you rent or borrow one for several days, shoot a lot of what you like to shoot, have a good look at the files, and print the best? then you'll have a rational basis to decide.
Well, I walk around NYC and make photographs. I'm influenced by William Eggleston, the photographers who were in the New Topographics show / book, Stephen Shore, William Christianberry, Walker Evans, etc. I don't copy their work, but it influences me... and I don't need to use the same tools (i.e. rangefinders or large format). I'd say street photography, but without people generally or where people are more incidental than the focus. Any camera works for my style if I want it to, but I want to have as little hassle as possible. My issue with the M8 is that the framelines are very inaccurate and I hate cropping. RF-ers are inaccurate in the first place but the M8 is a whole new level compared to film Ms. After using the X1, which is very accurate, the M8 won't do anymore. That is where the D700 comes in. It fits in my hand well, isn't crazy big, dials and buttons are in the right place for me, and full frame just makes sense when it comes to the history of Nikon's lens line-up. Most other DSLRs feel horrible to me after being a RF user for many, many years. The last serious SLR I used was a Nikon F4 in the 90s (which I sold when I discovered the Leica M). I no longer care about film and only use film because I enjoy using older Leica M cameras.
I love rangefinders (and will keep my M4), but my photography is changing. I now want the convenience of digital, clean images at high ISOs, and semi-accurate framing...and this is where DSLRs seem to thrive and the M8 only handles one well (it's digital lol). I also no longer care about having the smallest camera, just the best camera for my needs.
Renting for a few days is money that can be spent on 3 prime lenses for the Nikon. I understand the logic of this, but my wallet doesn't. Luckily, a friend has a D700 he may want to sell to me and said he'd let me try it out. Seems like the best way to do it.