My 2 pence
Hmm. I do like David's work, but I don't really think he compares favorably to Mark. David's style is very similar, like LuLu said very close to old-school Dire Straits, but I never saw the absolute mastery of poetry and the depth of emotion (that he can convey in that gravelly, understated way) that has been typically displayed by Mark. In a nutshell, the difference for me is that I enjoy David as background music, but I enjoy Mark when I'm giving the performance my full attention.
Do I find everything that Mark has done to be of the first water? Not at all. His body of work, however, and consistency over the years in delivering some of the very best the genre has to offer, has few equals. I admire his desire to investigate different sounds and styles; I loved his work with Chet Atkins, and would like to melt every last copy of I Want My MTV *grits teeth*.
David I again find consistent, but unchanging over time to any significant degree. He also seems (to me) to struggle at times with his lryics, sometimes falling back on almost cliche turns of phrase that may go over well enough from a rythmic perspective, but fit poorly with the flow of the song's message. Again, that's just my opinion.
FWIW, my all-time favorite for Mark solo is still Sailing to Philadelphia. For Dire Straits, the unbelieveable, uncomparable Love Over Gold.
Cheers!
Matty K.