Personally, I would choose a well-planned, small bathroom and a well-planned, medium-size closet.

Cater to closet space. As long as you have a well planned layout for your bathroom you will be set--you most likely won't find the need for more bathroom space in the future. Your storage needs will change over time, and inevitably grow. Empty space always becomes much smaller than you imagined once you install your closet cabinetry.

I agree with the above comments. The components of a bath do not really change much in size. So a bigger bathroom is just more space. Remember, usually a master closet is supposed to hold two people's wardrobe, not one.