I know a mix of credit is a fico scoring factor but more importantly history and util gets you the higher score. At this point I don't think a mix is a negative for you and if you don't have or need the debt don't get it. Concentrate on paying on time and your score will grow. You have cc in case you need it. If you have an emergency then consider the installment loan to cover it your credit is good enough to get one should a situation arise.