You can eliminate the D2 right away as the D2+ costs the same as the D2 and has BBE+ which the S9 has as well. That brings it down to two: Either the S9 or the D2+. It is now your call as there are various parameters, the first of which is the cost. If you are willing to shell out the extra cash for the S9, then go for it. I don't think that the sound quality should be any different between the two. The S9 has a stunning screen and looks uber-sexy

One thing to note is that there is no SD card slot on the S9. That may or may not mean anything to you. So, you decide.
I have a D2 16GB, and am very, very happy with it. In fact, it is the only DAP I ever loved after the Rio Karma. But, I recommend that you go for the S9.