Just to confuse matters further...

I wear a US 7.5 street shoe, US 8 running shoe...and have Women's Sidi Dominator's (mtb version of the Genius I believe) in a 39. They fit like a glove.

I have tiny narrow ankles and heels and fat piggies (toes) from years of running/childbirth.

This might be one of those situations where you really need to hit the LBS and try-before-you-buy.