I have to wear men's because I am a women's 11 and no one makes cycling shoes that big in women's sizes. But men's can be very loose in the heel, as SK points out. I guess the bottom line is to try on a lot of pairs and see what works.

Or, if you happen to have about a thousand extra bucks lying around, you can get custom made carbon shoes: http://www.lustracing.com/

I have a friend who has them and loves them. Whatever!