Michal I have owned both the D3 and D300 for about 7 months. I have taken both cameras on numerous trips and have compared both cameras with exact same lenses.
I have about 13,000 images on the D3 and about 3,500 on the D300.
I have not performed technical tests, just personal observations. In every single instance the images I produced from the D3 were far superior.
The metering seemed much better (closer to my desired result) and the colors are truer in the D3. The noise from 200 - 3200 seems very close to the same. but the H1-H2 images from the D3 are far superior.
In fact, I am seriously considering selling the D300. Not sure if I would move to the D700. Also, before getting the D3 my main camera was the D2Xs. I prefer the controls on the D3 over the positioning on the D300.