Have more letters written than you need. Do you work in health care? If not I would lean more to having a professor write it. People are busy. I went for the bigger fish theory (accomplished MD in Anesthesia) but regret it. Ultimately, I thought I could show this guy how badly I wanted it and that he would see it and convey that in the letter. In the end he flaked and I had to get a professor to write it.
So cover all of your bases.
This forum will make you hate other AAs. You would think that they were surgeons by their super