I am not too sure of a head-to-head comparison between these two. However, I know that the Nikon Dxxx (D100, D200, D300) range are more robust and better sealed vs. the Nikon Dxx (D40, D40x, D70 ... D90).
That's why I would favour the D300 any time.
Even though the D90 has better high ISO capabilities (not sure by how much, but it's supposed to be bit better), and is over £300 cheaper than the D300 .. still, the robustness of the D300 made me take it without a thought.
When the price was right, for me, I then moved from the D300 to the D700 (because I wanted, and needed, better high ISO capabilities).