Image-quality-wise there is no difference between the two, especially
no difference that, in the hands of 99.99% of the population, would
show up as a result of the camera rather than the photographer.
There is a difference when shooting high ISO RAW. It seems that the D300 applies less noise reduction when set to NR OFF, so when looking at RAW files at 100%, the D300 may seem to be a bit noisier at high ISO vs. the D90. What's happening is that it looks like there is less detail with the D90 at high ISO when NR is OFF which leads me to the conclusion that the D90 applies some Noise Reduction even when it's turned off. BTW...both cameras have Picture Control.
There is no doubt, the biggest advantage is the 51-point AF system. This is just great for sports any anything that moves around in an unpredictable manner (like little kids).
2. Ability to use the MB-D10 for portrait shooting
I personally don't like the MB-D10. Adds too much weight and I don't need the 8fps.
3. Ability to use old, great, cheap manual lenses
I have some old Nikon pre-AI that have not been converted. Unfortunately, they don't work with the D300. Since I have the equivalent in a modern design, it doesn't matter.
4. CLS Commander mode from the on-board flash, meaning you can have
off-camera flash for the price of an SB-600 instead of the price of
an SB-900 and an SB-600.
The commander mode is available for the D90 through Custom Setting e2. I have used the commander mode before when setting up an impromptu portrait session. Commander mode allows you to control multiple flash units (I use an SP-900 as the main light, an SB-800 as the fill light, and the SB-600 as the background light.
Whether or not that it worth $600 isn't something somebody here can
answer for you.
For me there was no question. I just wish the D300 was lighter but that's what you pay for a professional camera.
I'm also not happy with the way Nikon setup accessories for the D90. For instance, in the case of the GP-1 GPS attachment, you connect it to the D90 using the USB connector on the side of the camera. That makes the use of the camera kind of cumbersome because there is something sticking out the left side of the camera. With the D300, the GP-1 connects through the 10-pin connector on the front of the camera using a right angle cable, a much easier proposition to handle. The 10-pin connector is also weather resistant where the USB is not.
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Cliff