I have looked at the comparison between the D3 and D700 since they came out and I think I am going with the D700, and I really have a choice between either.
I went into my local Nikon dealer yesterday and they had a D3 for me to play around with and I asked them about going between the two and they just didn't know, not until they got one in their hands (they are getting 4 total on Aug 1).
What I found is and my running total of pluses and minuses (course this is purely subjective):
1. D700 has a self cleaning sensor unit, D3 does not (to me, needed)
2. D3 has dual CF slots, D700 does not (that is a cool feature)
3. D700 has monitor screen cover, D3 no (no biggie)
4. D3 has 100% viewfinder coverage, D700 has 95% (probably the biggest negative about the D700 to me, but I understand why)
5. D3 has 9-fps vs D300 8-fps (assuming grip, no biggie here, one single frame per second)
6. Shutter durability D3 300k vs D700 150k (never made it that high before trading a body in)
7. Voice memo on D3, none on D700 (to me, who cares)
8. Battery grip D3 built in, D700 buy (I love the built in grip)
9. Weight, D700 995g, D3 1,240g (for those of us in the U.S. who are mentally challenged at anything other than pounds, D700 just over 2 pounds, D3 just under 3 pounds, good difference)
10. Price D3-$5k, D700-$3k (big difference to me, if I go with the D700 I can get at least the 14-24 or 70-200 and still have change, so we are basically talking about extra glass, and really good glass at that)
11. Release Date / Asset value, D3-Aug 2007, D700-Aug 2008 (to me, constantly upgrade is a pain and expensive, I put this in there just because the D700 theoretically buys me a year of new development. When is the D4 out, January, June 09, September 09, who knows, but as soon as I buy it, it is sure to be on the way)
There were very minor differences not mentioned here because, to me, they didn't make a difference in my purchase decision.
So, if you add up all the pluses and minuses you get . It could go either way as the above is subjective. If I drop #11, the D3 has it by +1, so this is just my take on it.
I think I will probably take one of the 4 my local dealer gets and use both my D300/D700 combo. Now, if the new 70-200mm would just come out with the "N" coating that would be great (and make it more impossible to get).