IMHO the only reason to choose the D700 would be if you want/need to shoot at high ISO a lot. Although the D300/s is pretty good up to ISO 1600 it can't compete with the D700/D3. But at base ISO there's no real difference except a very slight edge in dynamic range for the D700. On the other hand if you're used to working with a cropped sensor and have lenses that you like, moving to FX could be a negative. The wide angle advantage of FX is vastly overstated IMHO. It used to be true, but there are a wide variety of wide zooms for DX nowadays.
Are there any really good wide angle lenses for the DX format, Jeff?
I tend to think, when choosing a format, one should always start from the perspective of the quality of available lenses for the particular format.
How does the Nikkor 10-24 (F3.5-4.5) on DX compare with the Nikkor 14-24/2.8 on FX? The 10-24 has a longer range and is more useful in that respect, but I get the impression that resolution fall-off in the corners is pretty bad.
The MTF 50 absolute resolution figures at Photozone cannot be compared across systems because different sensor resolutions are introduced into the equation.
However, the relative resolutions within the frame can be compared. On the D200 at 10mm and F3.5, progressing from the centre to the extreme edge, we have 2152, 1798 and 947 line widths per picture height (LWPH).
947 compared with 2153 is lousy. I don't think anyone could deny that.
The situation with the 14-24/2.8 on FX is much better, at the widest angle. The LWPH figures at 14mm and F2.8 are 3863, 3466 and 3033. Of course, they are all higher because the camera used for testing is a D3X. However, notice the much, much better edge performance in relative terms.
The frequent debates on this forum about MFDB image quality versus full frame 35mm also apply to DX versus FX, except that the jump in price, the increase in weight, and reduction in features is not as great as it is when moving from 35mm to MFDB.
As I see it, there are two main features that tend to distinguish the larger format from the smaller format.
(1) Lower noise and higher DR.
(2) Higher resolution with the same lens at the same F stop.
On the first point, there may be a few anomalies when comparing different formats. The D300 has about the same DR as the D700 at base ISO, but significantly more noise at 18% grey, according to DXOmark. This means, whilst the deepest shadows are as good as those from the D700, the rest of the image will exhibit more noise, at base ISO.
Above base ISO, even at ISO 400, there is no contest. The D700 has clearly lower noise, higher DR, better tonal range and better color sensitivity.
Combine these attributes with an 'effectively' better lens, then the gap widens.
However, here is where DoF enters the equation. Lens performance tends to fall off (at least to some degree) at apertures wider than F5.6, with most lenses.
This factor puts the DX or APS-C format at a disadvantage because the greater pixel density of the smaller format (at equal pixel count, compared with FX) requires a lens with higher resolution, not lower resolution.
However, this disadvantage of the smaller format tends to disappear at bigger F stop numbers (smaller apertures).
If an extensive DoF is sought, the larger format ceases to have a significant advantage, especially when one equalises shutter speed.
In other words, a 12mp D700 at F16, 1/100th sec and ISO 400, should have little or no image quality advantage over a D300 at F11, 1/100th and ISO 200.
If you disagree, show me the test results. I'd be interested.