Form what I've learned over the past few weeks, the primary difference between men's and women's cycling shoes is in the heel - women's shoes tend to be narrower at the heel so your foot doesn't lift up out of the shoe. Men's shoes are generally wider too, but I think the main feature is the heel.

I just got a pair of Sidi's - men's Genius 5 (?). After fitting me for shoes, my LBS guy thought the men's would work better for me than women's (I have a fairly wide foot). At first I thought they were too snug, and I was worried that after 10 or 20 miles in the heat, the shoes would feel too small - but I found them very comfortable on my first few rides. My heel does come up a tiny bit, but much less than it did with my previous shoes (men's Specialized).

I think a good way to go is to try on different shoes and see how they feel - particularly check if you can lift your heel out of them, and also that you have enough room in the toebox but not too much. If you can get a shoe fitting at a bike store where they understand how bikes shoes should fit, that would be a good way to go.