The Nikon D700 and Canon 5D Mark II are two of the latest, cutting edge DSLRs offered by the top camera manufactures. The key attraction to each of these cameras is the use of a "full frame" sensor. The term "full frame" references the dimensions of the old 35mm film that those of us born in the 80s (or earlier) still remember using. Digital camera sensors have typically been very small over the past several years, even most DSLRs have been smaller than full frame. The sensor size of compact cameras are tiny (about the size of a pencil eraser) compared to full frame sensors. The full frame sensors in the Nikon D700 and Canon 5D Mark II allow the cameras to take full advantage of light, which is crucial for smooth images. The Canon 5D Mark II and Nikon D700 have another thing in common - a hefty price tag. Both ring in at around $2700, though the Nikon D700 is starting to see a bit of a discount at some retailers. These cameras are not for the faint of heart, or the light of wallet. If you are shopping for one of these, then you should have a good understanding of photography and understand the difference that a $2700 camera can make in your images. There are pros and cons to each of these cameras when evaluated against the other.
Image Size and Quality
The Canon 5D Mark II, for instance, has a 21.1 megapixel sensor, which means you can make 20x30" prints all day long and not worry about needing more resolution. The Nikon D700 has a 12.2 megapixel sensor, which is plenty for all but the select few who do huge prints.
Low Light Sensitivity / Noise
Both cameras produce stellar images in low light. You can boost the ISO (sensitivity to light) all the way up to 25600, which was unheard of prior to the introduction of the Nikon D700's predecessor (the profession grade Nikon D3). Both cameras perform excellent up to ISO 6400 and then the images start to look noisy in both. I have successfully shot with each camera at night handheld and gotten relatively clean images. Indoor shooting without a flash is now possible in just about any light with the D700 and 5D Mark II.
I give Nikon the edge in autofocus capability. It has 51 autofocus points, whereas the Canon only has 9 AF points. Having shot with both of them, the Nikon is simply more sensitive in autofocusing, particularly in low light.
I hate to do it, but I have to say that it depends on you and your shooting style as to which one should win out the battle between these full framed beasts. If you are looking for speedy autofocus and don't need the huge files, go with the Nikon. If you want to print big, it's a no brainer, go with Canon. Obviously, if you are tied to a system with a bag full of Canon or Nikon lenses, then you have to keep it in the family. I don't think you would be too disappointed with either one. Me? I'm a Canon kind of guy, so I went with the Canon 5D Mark II - and I love it!