Nikon D750 is basically, the "wedding camera for the masses", competes now against the notably earlier Canon 5D Mark III. The D750 has the better sensor and the wider dynamic range. The Nikon D600 is available VERY low-priced used; the replacement D610 is a better-spec'd camera than the now-aging Canon 6D, which is an economy-based AF camera...11 AF points, 1 cross-sensor point...very 2005 Canon, as far as an AF system in the 6D. The D600-series Nikons have 39 AF points, not 11, and more cross-type sensors too. The higher end Nikons have a 51-point AF system.
"What full frame" is a tricky question. You can allow a 50mm lens to tie you to a future of Canon, or you could move to Nikon, or Sony, and those are different systems. I dunno...Kirk Tuck, famous pro commerical portrait shooter, REALLLLLLLLY loves the Sony system for portraiture. And he LOVES the 70-200 f/4 zoom they have...not the big, 2.8, but their f/4 model. he is actually a proponent of the f/4 70-200 lenses from Canon, and Nikon. Tuck has bought and used more camera system than any p;erson I know of, save for Steve Huff.
I personally LOVED the Canon 5D-III when I demo'd it. I liked the Nikon D4 as well, same day, side by side tests of those two. I ended up buying a Nikon D3x used, best camera ever made, for my uses.
I would strongly consider used, heavily depreciated bodies. Nikon D600,Nikon D800, Nikon D3x, D4, Canon 5D-III, Canon 6D, all bought used, with $1,000 or more right off the top for you compared to new. Right now we are in-between generations: the newest Canon high-MP cameras? Ugh...might as well by a Sony or a Nikon and get much better sensor performance for that type of ridiculously-high MP count.