I don't have a D300s, but do own both a D90 and D300.
Something I've been thinking about for quite a while, and putting aside the 'backup body' argument, is could I just sell both my bodies and get a D300s?
My own views of the D90 vs the D300 are that the D300 is a very nice bit of kit, highly customisable, robust, and nearly 8fps with the grip, but that the D90 has the better sensor with a higher dynamic range and of course has video (something which the D300s now has of course).
I purchased the D300 mainly for its fps capability for birds in flight/motorsport/aircraft shots, and the D90 because I fancied a bit of video. In hindsight, the video capability is something I haven't really touched very much, but I suppose it's a nice to have.
The D90 isn't exactly slow at 4.5fps, and I don't miss the 51 focus points of the D300 as I usually have the D300 set to 9 or 21 anyway!
Honestly, if I was buying again knowing what I use my kit for, I would be more than happy with a D90. If you had to force me to keep just two bits of kit (for some bizarre reason!), then it would be the D90 and Nikon 70-300 VR lens, which would cover 90% of my photography needs.
One area where the D300s will score over the D90 in my opinion is the ability to change important settings (ISO/Focus Points/Metering etc) using switches on the camera body rather than having to press a button and use a scroll wheel on the D90. A small thing I suppose, but one I find quite irritating when switching between bodies!
But, everybody is different, and whether you think a D300s is worth double the D90 will depend on what value you put on each cameras features.
My next move I think will be to change the D300 for a D700, but keep the D90 for the 'crop factor' and video.
Sorry, I can't help with the review website you seek, but I find the Nikon website as good as anywhere to compare the features. 
