There is a trend in the field that more and more job offerings for Ph.D.s are requiring that you also have you ASHA certification. In the new reality of economic problems (Universities are losing funding as well), many programs are looking for people with both an ability to produce research (Ph.D.) and an ability to work in the clinic and supervise students (Au.D.). If at all possible it would be the best move long-term for to make sure that you have both. It will allow you more freedom in the job market and will make you more attractive as a candidate. Obviously it has to work for you as well, but I think it is a very smart move to be as versatile as possible.