I went to the bike shop after work today and took the 48, 51 (women's and unisex) back out for a spin. I felt fine standing over the 51 and everyone's advice here was also very helpful. During the ride I felt a tiny bit more stretched out than I'd remembered it the last four times and it turns out the seat had gotten shifted a tiny bit back.

I also compared the geometry charts for the unisex and the women's versions and was surprised to find that the same sizes have the same dimensions for everything - top tube, stack, reach, etc. They said the main differences were the seat and the handlebar stem and width.

Anyway, the 51 felt fine to me. I asked them to order it in the color scheme I preferred!

Now how to go about making these next two weeks go by faster... hmmm