I'd go for the bigger cage. Females will often get longer and heavier than males, so need more floor space. I agree with Sauzo... Go for the shelf option (my boas LOVE theirs). Also, IMO, the sliding glass doors with the lock option are a very good idea. I like the idea of being able to keep people from messing with my snakes without my supervision and have also heard of some boas learning to slide their doors open.

I think the t13 was cheaper than the t10 for a while (not sure if that is still the case). If it is, I'd def. go with it. Your female may still grow, so it's always best to have more room. My boas are in T10s, but they are all male. If I was able to do things over again, I would have started with T13s instead just to give them and me more options. If my largest boa ever gets over 6', then he will have to get an upgrade to the next caging size... Right now he is pushing 5.5', so I'm getting there.