There's only really been 2 real revisions. The first one was the black driver which had a really dark sound, and some build issues. The second revision had the white housing and fixed most build issues. This is the one you want. The third revision was faulty and called back by Hifiman, so they went back to the second rev (or fourth if that's what you prefer to call it).
Don't expect any more revisions, the current rev is perfect.