It's close to the D7000 in a lot of the feature and a upgrade from the D90.
Not really. The D300s shares most of its features with the D3.
I used to own a D90 and currently use a D300s. The price difference between these two cameras (body only) is about $600, which is no small chunk of change. You can buy a nice lens for that kind of cash, or even a backup camera. The D90 is at the high end of the consumer models, while the D300s is at the low end of the pro models. While the D90 and D300s may sit close to each other in Nikon's lineup, they are really worlds apart.
BUILD
One of the biggest differences is construction. The D300s has a larger magnesium body, o-rings and seals to help with dust and weatherproofing, a stronger focus motor and shutter mechanism. You can actually hear the difference in the shutter compared to the D90. The overall footprint of the D300s is only slightly larger than the D90, but it's built like a tank.
FOCUS
51 points versus 11. The D300s has a lot more AF options too, in addition to better AF performance overall. The 3D tracking on this thing is nothing short of incredible.
DUAL CARDS
The D300s can store a SD and CF card, which gives you a lot more flexibility. You can shoot RAW on one card, JPG on the other. You can backup all your images by saving the same images each card, or even set the up to run in overflow for a huge amount of storage.
SPEED (FPS)
7 FPS with the D300s vs. 4.5 with the D90. With the Nikon vertical grip (MB-D10) and different battery, this jumps to 8 FPS. If you shoot sports...this combined with 51 point 3D tracking...forget about it. This was one of the major deciding factors for me, as I shoot a lot of sports. We're talking about nearly double the frames here.
MODES
There are no "idiot modes" on the D300s. Say goodbye to Auto, Sports, and the others. This can be a good thing or bad thing, depending on your point of view.
There are a lot of other upgrades too, but the above are the major ones that will put hair on your chest. It's easy to get caught up in all that, but remember that it all comes with a hefty price tag. For me, it made sense to upgrade considering what I shoot. The D90 is a fine camera, I loved mine and would probably still have it...if it hadn't been destroyed in water. You have to ask yourself if you really
need or even want all the extra stuff that comes with a D300s.