How much customization is on the nikon d5100?

In manual could i set up custom white balance and things like that?
how does the customization com pair to something like the d300s?

You can control absolutely everything

the difference between the two is the build quality (the D300s is better), buttons (you don't have to surf menus to change settings on the D300s, whereas on the D5100 you do), and the fact that the D300s has an AF motor

the D300s, however, is getting aged and should be replaced this year. The D7000 is better than the D300s and is cheaper. Look at that instead of the D300s

What exactly do you mean by customisation? Cameras like the D300 which is aimed at semi-professional and professional users tend to have a lot more user options to allow the camera to be more effectively used. I can't say exactly what, but usually you get "softkeys" which can be programmed, and the D300 can be programmed with lens information.