I will compare and attempt to review the two Nikon cameras I own. There's has been discussion comparing the D90 and D300. As far as which is better...that will be up to the user.
This is the most obvious difference with these two bodies. The AF system is noticeably different. The D90 if I'm not mistaken has the multi-cam 1000, The same as the D200. The D300 has the the multi-cam 3500DX, which is essentially the same as the D3 with the exception of it having the DX rather than FX suffix. The most readily noticeable thing is the number of focal points. The D90 having 11 as opposed to 51 but his is common knowledge. The two biggest things I noticed is that while the D90 is faster and more accurate than the D40 (and therefore the D40x and D60), it's somewhat slower than the D300. The focus is also a bit less accurate. In other words, it won't "do so much of the work" for you. The 3D tracking is a bit fussier as well but these are things that can be overcome with skill and patients. The other thing missing that I noticed was the ability to set the AF system to lock if focus was not achieved. The D90 will fire away, giving you beautiful bokeh filled shots with no discernible subject lol. The AF sytem on this camera is by no means a handicap, it just requires a different technique.
These are much the same as in the D300. You have the same standard and vivid modes with the same saturation levels. With the D90's slightly newer sensor, it perhaps handles colors a bit better. One thing I often hear people say is that their D90's often seem to have better dynamic range than other cameras, even their D300's. This really isn't the case. Most users don't tweak around in their menus. In the picture controls, the D90 has two extra settings for Active D-Lighting; very high and auto. The factory default setting for this is auto so most people never change it and perhaps there's no need to. This feature works splendidly in auto mode. Most people don't know how to use this feature, the D90 solves that problem. Presumably, the D400 will too.
As mentioned by some on the web, is slightly better overall than the D300's. When post processing files, less noise is seen and less noise reductions needs to be applied. it also seems to retain a bit more detail at higher ISO's. ISO 1000 is actually not bad at all IMO.
Video: It's there if you want it. I've played with it for no other reason than this. It's easy to engage; just press the LV button then press ok and it starts recording. Press LV again to disengage and stop recording.
Another no brainer here. While not built like a tank like the D300 and D200, because it's a Nikon, it's still feels pretty solid to me. Lighter in weight too but when it comes to weight, I'm a poor person to ask as carrying around "heavy" gear isn't an issue to me. The placement of the buttons is another obvious difference but I found that it's pretty easy to get used to. The only one I have an issue with is the ok button not being at the bottom left hand side. I frequently press the + sing button lol. It's does nothing though unless you're viewing a pic. The other weirdness is that the fn and DOF buttons are reversed. It's ergonomically smart however; the DOF button is right where my ring finger is and the fn button is where my middle finger is. Unlike the D300 where I shoot with my middle finger, I can use my index finger for the shutter. Using my index finger isn't a problem, it's just different than on my D300. Certain things are in different places, it still works out in an ergonomically smart way. Perhaps Nikon thought about us guys with large hands and long fingers. I thought having shared buttons would be a problem, it's not. The info/WB/lock button for example will go to WB anytime. The only exceptions are let's say you're in a menu and want to know what something is. Then it become an info button. If you're looking at a shot you took and press it, it will lock or unlock that shot. It's similar for the other two button below it. It's a really smartly designed camera! There are four other buttons on top that work in much the same way; you press them and spin the main command dial to adjust. Unlike the D300, it has a scene mode dial in case you hand it over to a non photographer friend of yours. You can just set it to "dummy mode".
So what's the verdict?
Which do I think is better? Neither really. Each one is designed to fit into the niche it was meant for. They are very similar in the way they can make an image look. Same basic technology as far as picture controls are concerned. So for me, the D90 is my summer fun camera and the one I will likely use for most of my portraits and long exposure shots and scenery. The D300 is my more serious camera I will use during the winter birding season or when I head out to the mountains or forest. I got the D90 to compliment the D300 and it was a good decision. They make a great pair.