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I was considering both these choices in late '09... I chose the D700 over the 5DII because the D700 was best-in-class regarding to two things that were very important to me - 1) AF lock speed and 2) Low-light/High ISO capability.
I shoot literally everything, but the subjects that are most important to me move around a lot in dim lighting, and I wanted immediate, always-reliable AF lock and very good noise performance.
The shot below looks 'easy', but it wasn't, the exact moment only lasted literally a second or less, and it was dimmer there than it looks - but the D700 + 50 1.8 locked
immediately, and at ISO 1800 I don't even have to consider the use of noise reduction:
(Older boy had gotten hurt while running around the house and was crying - his little brother often 'consoles' him when he cries, but it only lasts a second and I never seem to have a camera when it happens - until that time )
For me, moments like that ^^ are just excruciatingly frustrating to miss due to AF hunting.
The dynamic range + DOF control at 'normal' focal lengths bring something extra to your photography:
(although the D700 & 5DII Dynamic Range is now exceeded at base ISO by the D7000/K-5)
The Nikon D3s was perhaps about the same with AF, better in low-light, but it's not really the same 'class' as the D700, it's a huge $5200 camera, and the size & price didn't appeal to me.
The 5DII had a lot of resolution and very good video capabilities, but neither of those things were important to me personally, although I completely understand how they would be important to a lot of folks. And that new $2K price is pretty sweet, I have to admit
But D700 was a no-brainer for me.
I've been eagerly anticipating the D800 for a while, but the rumoured specs are starting to
maybe make me think I'll be better off sticking with the D700 for now. 36MP is just overkill for me - it represents not much of a jump in low-light capability + a slower and more cumbersome workflow, because of the huge files. If it were a $2300 body I'd probably bite, but at $3000 - $4000 (rumoured,) I don't know if it's an appealing enough upgrade over the D700... Judging by some of the scuttlebut on the Nikon fora, others may agree with me. D700 still resides in a coveted sweet spot - excellent in some very important areas, relatively affordable. We'll have to see exactly what the D800 turns out to be and how it performs though, or if we'll see a D700s appear as well (essentially a D700 body with the incredible D3s sensor.)
(And then there's the outside possibility of a Pentax FF in 2012 or 2013, throwing grease on the track...
)
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