Given that the medium frame has a horizontal top tube length just 15mm longer than the small frame, why do we think the OP is going to feel any difference in reach or stretch-out between them?
But when she slips and bangs her public bone on the top tube, I reckon she'll feel that.
Possibly you can make do with the medium, but there is no downside to getting the small, except for the salesman who wanted to make the sale ASAP.