Although both Nikon D90 and D300 are made by the same company, there are some differences between the two. In terms of price, D300 is quite expensive compared to Nikon D90. This is not always an issue since the decision to buy any of the two depends on one's budget.
Another difference is that the body of the two cameras is made with different materials. The material used in making Nikon D90 is polycarbonate, whereas the body of Nikon D300 is made with magnesium alloy. This makes D300 be lighter compared to D90. Unlike D300 that can be used to process 14-bit images, Nikon D90 can only be used for 12-bit images.
Also, unlike Nikon D300 that supports Compact Flash memory cards, Nikon D90 uses SD memory cards. Another difference is that Nikon D90 comes with more additional features that are absent in Nikon D300. For instance, more image processing options are available in Nikon D90.