Does the 5D have basically the same performance as the Nikon you're talking about?
D700 has "better" low light performance (and many higher sensitivity settings) than the 5D, and just a little bit smaller of an image. But the 5D is no slouch, especially for what they sell for on the used market.
Being able to use AI and AI-S lenses makes Nikon a no brainer for what I shoot and for what I want to invest in lenses (i.e. not a ton of money), and that is why I went with a Nikon recently after shooting Canon digital since 2003. I held off for a long time because Nikons had such horrendously foul high ISO performance until the D3, and I shoot mostly at high ISOs when using digital cameras. But I didn't invest heavily in Canon, so switching is not that hurtful to me. I had only one lens remaining (50mm f/1.2), which I sold to a friend.
D700 also has better build quality than the 5D. It is built pretty much like a pro body, while the 5D is built like a mid-level consumer body. This being said, I have almost 8 years of hard use on my Canon 10D, and it has never failed me entirely. The shutter button is a little loose, and has to be somewhat stabbed (or hit at
just the right angle) to fire a shot now, and the pop-up flash no longer synchs, though it fires. The shutter button bothers me, and I will get it fixed eventually, but I never used the flash, so forget spending any money on that. So if I am hard on cameras and a consumer body has for the most part lasted me fine for 8 years, I am not sure that this difference in build quality should even be a factor to those who are not extremely hard on their cameras. But it is something to appreciate and to help put you a little bit at ease...and it just
feels better to use.
5D
Mk. II (I was previously discussing the plain 5D) trumps D700 in image size by a factor of 2. That is its only real superiority the way I see it, though very few people would actually find this huge file useful/necessary. The original 5D, as mentioned, only trumps the D700 by about 1 Mpix.
I'd say if you are comparing D700 to 5D, D700 wins on all accounts except price. (5D's cost about half as much on the used market.) If you are comparing D700 to 5D Mk. II, D700 wins for everything but landscape photographers, or other photographers who don't need blazing auto focus, don't really shoot a lot of moving things, and don't need to work fast or be hard on their cameras with day-in, day-out use, but who need the most beautiful prints you can get from a small format digital, with the option of going
very large. The minutiae of high ISO "quality" is debatable between the two, but both are close enough to be considered equal in all practicality, IME. (I have shot both, though I have not owned either...until a few days, when my own personal D700 arrives.)