I highly recommend the women's style packs! Especially if you are at a disadvantage with height. I have used the under-bike bottle cage but only for extra water I use while stopped and have the ability to wipe the top before eating mud (some of my rides can be up to 8 hours with no water supply) If you have the right cage it will stay just fine and not get in your way...just me I tried to hit it on logs many times!!!

I would NOT use a behind the seat cage on a mtbike if you plan on going down hill at any time...gets in your way when you are "getting behind the saddle" to avoid "going over the bars". I do know of a few guys who have them but their seat posts are so long that the bottle sits well below the seat.

Another option used quite a bit around here is a bottle in the jersey pocket...