As someone who knits ALL socks on a 9" circular needle all the time I can tell you that 9" is preferable over 12". A 12" needle is too long to accommodate the number of stitches needed for a woman's sock. Even so, if you don't use the 2 circular needle method you will still need DPN's for the toe decreases. The 9" circular needles that are made for sock knitting have short pointS..both being shorter than the norm you will find on standard circulars. This makes the needle more flexible and easier to use.
I had always used DPN's to make socks until the past year or so, when I found these wonderful little circulars.