I have experience of neither!
But the D90 shares the same sensor as the D300 so it should have pretty much the same image quality. It probably has better JPGs straight from camera as Nikon will have tweaked the image processing algorithms in the time between launch. Anyway there’ll be nothing significant in it.
So the big difference will be in usability. It’s up to you to decide whether the price difference is worth it.
Personally I wouldn’t upgrade at all. There’s no way I’m forking out to upgrade every generation of DSLR. I’m still on the D70 and waiting another generation at least before I change it. I’m interested to see what the successor to the D300 is like though.