Image quality and basic performance are the same. I remember people talking of improved AF performance but I have not found any AF differences that are significant in real world shooting.
I do use the D300s as my primary body. I hate the multi-sector on the D300. The separated center button on the D300s is much better. I also like the D300s battery door that slides open without a latch lever.
They removed live view from mode dial and gave it a dedicated button on back right next to the Info button.