I think Quirrel used a powerful killing curse. I think that this is the case mainly because I don't think a stunner would have had as much effect on a patronus as the spell which Quirrel used; also because, even when facing a significant decline in his reputation in Harry's eyes (in hostile territory) Quirrel did not think to try to claim that it was a green stunner, leading me to the tentative conclusion that Quirrel had not thought of a green stunner at that time.