Basically, the new D300s is exactly the same camera as the D300 in terms of features, except for the following:
1) D300s shoots HD movies at 720p resolution, 24 FPS with stereo audio. Maximum length is 5 minutes for 720p and 20 mins for lower video resolutions.
2) D300s is slightly faster than the D300, shooting 7 FPS in Ch mode (Nikon D300 is 6 FPS). With MB-D10 battery pack, it will shoot 8 FPS.
3) A new release mode “Q” (quiet shutter-release) is added to the dial right after Ch (continuous high speed).
4) Dual card slots – the Nikon D300s features dual card slots to work with both CompactFlash and SD (SDHC-compliant) cards. Either card can be used as the primary card. Secondary card can be used for overflow or backup storage, or for separate storage of NEF (RAW) and JPEG images and images can be copied between cards.
5) Active D-Lighting now has “Auto” and “Extra High” added. “Auto” is something expected, as both D700 and D90 have this mode. The “Extra High” is something new though.
6) Nikon D300s is slightly heavier than the D300, adding 15 more grams of weight, weighing total 840g total.
7) Nikon D300s has a dedicated “Lv” (LiveView) and “Info” buttons on the back of the camera.
8) Nikon D300s has a virtual horizon now (D300 did not).
Is D300s worth the upgrade? If you already have a D300 and do not care about the video feature (which kind of sucks, since I was expecting full HD at 1080p), it is not worth the upgrade. But if you don't have D300 it's worth it to have one. Which I plan to purchase one unit for myself. Before I do that, I need to check with my Chief Finance Officer (my wife) hehehe.
The sensor of the new D300s is basically identical to the older D300. It is nice that the D300s has dual slots and faster frame rate, but it is nothing extraordinary.