Color looks similar to me.
I find the AF system in D300s to be slightly faster and a lot more accurate than the D90. That, plus 51 AF points, which can get overwhelming sometimes. But you can control number of active AF points according to your needs. D300s also allow you to save settings set for camera and custom menus (separately). Allows me to save my settings for different shooting situations, and I can call it up very quickly. The most useful feature for me is the ability to meter with non-cpu manual focus lenses.
D90 is more the size of the D70. It actually holds better than a D70. I like the D90 built too, material used is nice, plastic, but no plasticky feeling. feels rock solid.
D300s is quite a bit larger and heavier. Magnesium alloy body, and weather sealing. Thing is the D200, D300 are notorious for the rubber grip material coming loose and peeling. So if you do go the D300s route, be mentally prepared that it will happen soon (most report 1-2 years).
And also, I do not believe in grips. I like to travel as light as possible.
But for image quality, I have to say they are identical.