As others have already said, the D7000 is a more capable camera in just about every way. This is especially true with respect to video, which is where the most significant advances are currently being made in DSLR technology. From everything I've read, the video capabilities of the D7000 far outstrip those of the D90. Even for casual video use, that by itself may make the difference.
The 18-105 lens is an excellent place to start. Its range of focal length is pretty close to your S600 -- with some more at the telephoto end -- so you pretty much already know what to expect.