I have read on many sources that it is fine for women to use the 5% preparation. (As long as you aren't pregnant or nursing, which I suspect is likely not the case for you anyway, either concentration is fine for women.) In fact, I had read that Dr. Rassman recommends that all of his patients, both men and women, use the 5% formulation as it produces better results.
The reason behind recommending the lower concentration seems to be as a result of the potential
for some women to have unwanted facial hair growth when using the 5%.
On another note, using the foam is preferred over the liquid solution because apparently styling is easier as it is not as oily.
I have only ever used the 2% solution (for men) but stopped it rather quickly as I did not like the way it made my hair feel. Apparently the 2% women's solution has an alcohol base which is supposed to make it less oily than the men's 2% solution which has an oil base. Perhaps had I tried the women's I would have had better luck and stuck with it.