Are you unable to opt for getting paid for the position? I hold a TA position at my school and we have the option of receiving credit or pay for the position. In my case, receiving pay was the better deal. Doing it for credit was basically like paying the school to work for them. In addition, if I were to take it for credit, it would be graded on a mandatory credit/no credit basis instead of letter grades - wouldn't help my GPA, so there was no incentive to electing this option.
However, if you can only choose between doing it for credit or merely counting it as volunteer work, then doing it for credit may be the better bet provided that you are not paying by the hour. If you will be part time and paying for each credit hour that goes on your transcript, then it is not worth doing it for credit. You should also check to see how it is graded - does the professor actually assign you a letter grade based on your performance, or is it mandatory pass/fail / satisfactory/unsatisfactory / credit/no credit?