Well, I just had to chime in here because I have used both of these cameras and been in your shoes not to long ago. A few months back I sold all my Canon gear to go full frame and all the threads in photo forums etc... seemed to sway me to the D700 so that is the direction I went. This was not very smart of me for several reason but mainly because all I had ever known is Canon, I had never even held a Nikon (well only once). I had heard all the hype over the D700 and how amazing it is etc... so I just knew that I couldn't go wrong. At the time I switched there were several other ladies switching and they were loving theirs so I just knew I would too. I had also read thread after thread on the photo forums about how people hated the focus on their 5DMii so those all scared me to death of it. I sold all my gear within about 2 weeks and bought a refurbished D700 from the Nikon site. I finally got it and snapped a couple pics and the first day I thought I was really going to like it, then it just all went down hill for me. I tried every focus combination under the sun with it and still had HORRIBLE focus, I also had this really weird like glow in my outdoor photos, the 50mm lens I tried it with caused HORRIBLE vignetting and I know that was a good lens because the person I borrowed it from rocks it. I also had the worst time with metering on it. All those factors combined made me package it back up and send it back to Nikon at the weeks end. I will not even go into the HORRIBLE experience I had with Nikon's customer service because that would take to long.
I went for over a month without a camera after this because I was still scared to death to buy the 5DMii. I really didn't know which road to take but finally I just decided that I was going to try the 5DMii. This is where I get all giddy inside because the moment I took it out of the box I was in LOVE. The minute I snapped the first picture I was even deeper in LOVE. I wish I could describe the beauty of this camera. I really feel unworthy to even be shooting with it. The only thing I regret is not buying it from the very start. All of the things I worried about with focus and such were not there. I will say that I loved the way the D700 toggles focus points but that was the only thing I liked. The bokeh on the mii is so much creamier and dreamier than it was on the D700. It's honestly smooth like butter. I had heard people say stuff like that but I saw it first hand for myself. In both the D700 and 5DMii, my RAW files come out so much better with so much more clarity and detail and color(minus all the focus problems I had with the D700) than they did on my other cameras and I know that has to do with full frame. It's truly amazing!!
Now, having said all that I realize that I may have been and may still be Canon biased. Like I said earlier Canon is all I have known. Maybe if I had stuck with the D700 longer I would have fallen in love, who knows. I also realize that I could have gotten a dud with my refurbished. It only had 350 clicks on it but there was still some really weird stuff going on. I will also say that there are equally amazing photographers shooting on both sides of these camera brands and I think you just have to find the one that fits you. I would definitely suggest you hold and even shoot with both of them, if possible, before making a decision. It's definitely not a cheap one to make. I just knew the minute I took my Mii out of the box that I am a Canon girl. It's familiar and I knew exactly how to work it. That was part of my frustration with the Nikon I am sure. It was foreign to me. I didn't know where anything was. I am glad I came back to Canon because the lens choices are so much better. I really didn't know what direction I was going to go with the Nikon. Canon offers a little of everything and for every budget. I really wish you luck in all this. It was truly an agonizing decision when I was going through it all and I am thankful it worked out to the good in the end because I was a little worried for awhile. I am hoping yours will go smoothly and you find a new camera that you love, no matter what side you go with. As long as you love it, it will be the perfect one!!
I was happy with mine too until I saw the difference in the images between a DX and an FX. Now, I just have to have one!