Polite Way to Turn Down a Job?

I interviewed for a position last week that I wasn't sure that I wanted. At the end of the interview, the supervisor let me know that she was interested in hiring me. I knew that I was going to have another interview yesterday for a job that I was really more interested in, so I told her that I did have another interview and she asked me to call her after my interview to let her know what I had decided.

My interview yesterday for the job that I really want went very, very well and the person who interviewed me called this morning to offer me the job and I accepted. However, I have not called the lady who interviewed me last week back yet to tell her of this decision, because I don't know a polite way to say, in effect, "Thanks, but no thanks." One of the reasons why I am turning this position down is because it is for a very, very small (census of less than 10 patients) hospice company and the job that I accepted gives me a lot more stability because I will be working for a very large hospital system. Would it be inappropriate to tell her that I was concerned about the company being so small that I wouldn't get many hours (it was a contingent position) and I needed more stability than that? What exactly would you tell her? I just hate to call but I promised her I would and I can't break a promise.