In my opinion, it's not worth it. There's just not enough features to justify the price difference. Wait for D300x (or whatever they'll call it :greenpbl

. Maybe the next model will offer more megapixels, and certainly more features.
I doubt there will be a D300X. A D400 in about 18 months sounds about right though.
The D3000, D5000, D90, being entry level cameras, all share Nikon's entry level Multi-CAM1000 autofocus module, having 11 focus points, only 1 of which is a cross-type focus point.
The D300/D300s has the same, 51 focus points (15 cross-type sensors) Nikon Multi-CAM 3500DX autofocus module; AF fine adjustment possible, that a D3s or D3X has.
Add 9 auto brackets (D90, only 3), up to 8 fps burst mode (D90, only 4.5), 1/8000 shutter (D90 only 1/4000), uncompressed RAW (D90, compressed RAW), 2 memory cards (D300s, not D300), Hi-speed USB, NTSC, HDMI, PAL, 10-pin Terminal,
HDMI Type C interfaces (D90, only USB, NTSC, HDMI),
metal body and weather sealing.
Those added features are what separate an entry level camera from an advanced amateur camera.
If you don't need any of the additional features.....don't spend the money for the D300/D300s, because the D90 and the D300/D300s have the same image sensor and EXPEED image processor, so the image quality is virtually identical.