Since the D90 hasn’t yet been released and I haven’t even seen one, this comparison is purely based on the specifications that have been released by Nikon. Base your purchase decisions on the proper reviews, once the camera has been released. In the meantime, read on for fun and at your own risk…
The big difference between the two bodies is that the D90 offers a new HD movie capture mode with mono sound. This will open up new opportunities for creativity (I'd really like to try using my 10.5mm fisheye for this). You could make a short movie clip and combine it with still images for presentation on the web with a program like Soundslides or on YouTube. Score one big one for the D90, zilch for the D300.
The downside is that the D90 requires a mono microphone that may render it less weatherproof than the D300. You can see the mic as 3 small holes just above the D90 badge. If the movie mode doesn’t appeal to you, then this is just one potential area of weakness that the D300 doesn’t have.
The D300 weighs 825g and the D90 620g. This is a pretty substantial 205g weight difference. I’m more willing to accept the weight penalty to carry the D300, given its more robust build quality and weatherproofing, and the places I go and conditions I subject my equipment to.
D300 is 100%, D90 is 96%.
The D90 uses the 11-point Nikon Multi-CAM1000 AF module, same as D80, and D200 but updated to include 3D focus tracking and Face Detection (of the D300). I prefer the clean look in my viewfinder with the D300’s 51-point AF system, and its greater coverage works better for me.
Nikon rates the D300 for 1000 shots per charge (CIPA). Surprisingly, the D90 gets only 850 shots per charge.
I’ll stick with my D300. For the person who doesn’t challenge his gear in environmental extremes, the D90 would be a good choice. It’s lighter, more versatile, and offers the image quality of the D300 for less money. For the person who travels to rugged and remote locations in hot, cold, wet, dusty (tick all that apply) environments, the D300 may give you more confidence as it is backed by Nikon’s reputation for producing reliable and durable professional grade cameras.