I have heard that the WSD (women's specific design) frames are only really needed by short women (around 5'2" or shorter). Taller women can be fit into normal bikes (men's bikes) by adjusting the seat post and swapping out the handlebar stem to bring the handlebars back closer to the saddle.
I would avoid the mixte frames or step through frames. Who is going to ride a bike in skirts these days? Also the mixte frame is heavier than the diamond frame. I would also think that economies of scale (much larger production) of men's frames would make a comparable men's frame much cheaper than a mixte frame.