The D300 body has a more comprehensive set of features, and if you need those features then you need them. However, the D90 has benefited from a year of extra sensor development, and Nikon has put that year to good use and significantly improved on the sensor. When the D300 successor is released late this year or early in 2010, it will leapfrog past the D90 again.
Not true. The sensor in the D300 is better even though it is a year older. Check out dpreview who did a 34 page review on the D90 and in it compared it to the D300. Their review of the D90 is the most extensive I have found.
Here are the differences so lets end the speculation:
Quote from Digital Review, D90 and D300 comparison:
-The Nikon D90 offers 12-bit image processing compared to the Nikon D300 were 12-bit or 14-bit can be selected; internal full 16-bit.
-Nikon D90 Body is made of polycarbonate versus the Nikon D300 which is made of the superior rugged magnesium alloy and features enhanced weather seal protection.
-The D300 provides faster start up (0.13 sec) and minimal shutter lag (45ms) compared to the D90 (0.15 sec start up and 65ms shutter lag).
-The D300 offers phase-difference detection Auto Focus in "Tripod shooting mode" (not available on D90).
-The D90 incorporates a 420-pixel RGB sensor for metering compared to the Nikon D300 with a higher end 1,005-pixel RGB sensor. Leveraging the data provided by the1,005-pixel RGB Matrix Meter, the Scene Recognition System in the Nikon D300 provides more precise subject identification, optimizing autofocus, exposure and white balance detection.
-D300 offers both a CPU and AI exposure meter coupling to support metering with older type Nikkor lenses.
-The Nikon D90 features 11 focus points versus the Nikon D300 with a broader AF coverage with 51 focus points.
-The Nikon D300 uses a faster AF module (CAM 3500DX) and offers 15 cross type sensors, Fine AF adjustment is possible on the D300 to match the lens in use.
-Viewfinder frame coverage of approx.96% with the D90 compared to 100% coverage with the Nikon D300.
-The D90 offers Continuous shooting at approx. 4.5 fps versus approx. 6 fps with the D300. The D300 can shoot at up to 8fps with the optional MB-D10 vertical battery grip accessory.
-The D300 offers a greater degree of customization total of 48 custom settings.
Actual pictures (RAW):
http://www.dpreview.com/reviews/nikond90/page33.asp
Now it is up to everyone considering these 2 bodies if the price difference is worth it for the better D300. For me it was.