I usually shoot in a fast pace & complex lighting environment. These are the reasons that I choose D700 over 5D MkII:
+ Much better AF performance. On D700 it's like "Sir! yes sir!". On 5DMarkII it's like "Uh oh. Alright..."
+ I'm an expo disc user. With Nikon I can quicky change my custom WB with a press of button. For 5DMrkII you have to go through quite a number of steps.
+ Auto ISO in M mode as mentioned by surrephoto above. Usually my setting is ISO 400 with Flash and Auto-ISO without Flash. I can simultaneously change between these 2 modes by the Custom-Function button. This is a killer's function!
+ Built in flash: it does help when your main flash fails, or you need to control other speedlights.
+ Built-in Focus assist beam. All my friends who shoot Canon always bring a touchlight to shine on the model when doing low light studio shoot.
+ Nikkor 24-70 f.28 is much sharper & faster AF than Canon's version at f/2.8. This is the lens I use most of the time.
Nikon/D700 does have its cons:
+ No movie
+ Lower Resolution. Sometimes I do need to crop my pictures into better compositions.
+ Staight out from the camera, 5DMarkII gives better asian skintone, more contrast look than D700. Shooting angmoh, 5DMarkII gives too much reddish skintone. All of this you can adjust by in camera setting or PP.
+ D700's dynamic range is not as good as 5DMarkII. Thus transition between brightlight and its surrounding is not as good. Btw, Sony DSLR has the best Dynamic range
+ MB-10 Grip is kinda sux, The shutter button is pretty sensitive, I misfired quite a number of time with it. But if you put a D3 battery into it, i feel it made my D700 more responsive and quiet shutter sound.
+ Limited lens options compared to Canon. Recently released Nikon lens are quite disappointed compared to Canon's too, especially the 70-200 VRII & 85mm f/1.4.
In fact I'm waiting for the performance of the upcoming 5DMarkIII and perhaps 24-70 f2.8 MarkII (just patented), then I'll decide if I wanna switch to Canon