LORs are not all professors

[jlbozoglan]

I need 3 LORs, and I already know 2 I'm definitely going to use:

1. A professor I had for 3 classes, and that I was a teaching assistant for twice.

2. A researcher (PhD) in my school that I was a research assistant for. (though i'm going into the medical field)

I am currently volunteering at a hospital (i am going for my MSW, but at the hospital, I volunteer with geriatrics), and my supervisor said she would write a letter for me, but should I try to get another professor instead? I know she'll write a great letter, and the other professor I'd use is someone I had twice and don't really know personally. There's also an older grad student who said she's write a letter for me, but I think my supervisor's letter would weigh more with the grad schools.

Who do you think would be the best candidate for my third LOR?

Thanks!