Well, it depends on what you'd prefer or what you use it for. Quality of picture is about bang on between the D90 and the D300/D300s. The D300 just has a much better build of body so you can have more confidence in it surviving a knock or fall. It also has a higher FPS and more focus points/sensors. But it's also more expensive, and it's heavier (weight can be considered both and advantage or disadvantage). The D90 has video mode if your into that. And it's cheaper.
So... your choice is:
D300 for a better build, 51 auto focus points, and 8 FPS with a battery pack
D90 for a ligher camera, money towards a lens, and video
Choice is yours!