W has larger bust to waist ratio, and larger armholes. Pants have a larger waist to hop ratio. It depends on your proportions which fits better.
For me, a 42H on top, it's women's sizes all the way. Women's petite pants fit me better than women's because the crotch length is shorter. (when seated, that's the distance from your waist to the chair. When it's the right length for you, the crotch won't be half way to your knees, it will be up where it should be - making you look smaller)