To here previews of the Oblivion music and clips from Morrowind, click here.
My opinion: I really liked the Morrowind music, but I think that this stuff is much better. The music in Morrowind was a little bit repetitive and contained to much brass in many places. The Oblivion stuff has a wider variety of instuments in more places, which makes for better harmony and themes. This game just keeps getting better and better!
Based on the parts of the Oblivion soundtrack I've heard, I think I'll like it better.
I will, of course, copy the better Morrowind music files into the Oblivion folder after making sure that won't be resulting in two very similar tracks.