This thread is long-over but fresh as new fallen snow for me, who has loved SD and Donald and Walter's solo work since the late '70s and recently has chosen to go down the WB rabbit hole a bit...
Comparing Walter's solo work to Donald's: I echo everything everybody has said before of course. What I find distinctive between the two is in the musical sensibility; Donald's musical inspirations so clearly come from jazz, a particular sort of New York jazz that really doesn't incorporate other world music styles and his sound emulates that. Walter to my ears appears to have been listening to a lot more different material from all over and chose to pull that into the material he wrote and performed.
Lyrically, the characters the two write are quite different. Donald's characters are soft focus, a bit above the whole thing. I never imagine those characters dirty or puking in a gutter or getting into a bar fight. Walter's characters seem to come from a seedier, more interesting neighborhood. They're a little bit dangerous and as a result their stories are more interesting to hear, if a little difficult sometimes.
I think the music and lyrics work together for each of them in their own ways but if you count listens to solo work between D & W, Walter wins in a walk with me.