So true! It is better to get her a bag that you think might be to warm. It will probably be perfect for her.
Greetings from a woman that just ordered a sleeping bag intended for -40 degrees.... (Celsius).
The difference between a Women's bag and a Men's bag is the temp rating. Typically men's bags are sold based on their T-Limit rating, and women's based on the T-Comfort rating. Take for example the Mountain hardware Laminina 35 and Lamina 35.
The Mens bag has T-Limit - 0°C, T-Comfort - 5°C. The Womens version has a T-Limit - -4°C, T-Comfort - 2°C. Both bags are sold as having a 35°F/2°C rating.
On the 20°F/-7°C Laminina 20 and Lamina 20, the ratings are T-Limit -12°C, T-Comfort -7°C or the women's, and T-Limit -7°C, T-Comfort -2°C.
In both cases both bags weigh the same, they achieve the extra performance by making the women's bag shorter.
I do wish people would stop using the whole seasons thing for measuring sleeping bags, it's a subjective measure of utter uselessness. We have a standard by which you can compare bags, lets use it.
Arya, OOI, what bag did you get for -40°C ?
J