Just from personal experience, I'd go with pants. I've seen a lot of women that ride in the traditional dress, but it takes a really well trained horse to tolerate the drape over their bum. I've ridden in a dress, and it is a bit odd at first.
Also, I'd advise if you're going with the dress, make your own. Even if you wear a button down shirt tucked into a high waisted denim skirt, it would look traditional. You can find denim-type fabric at Wal-Mart, and if you need a pattern, you could probably find on there too. I advise wearing pants or shorts under the skirt though.
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Bailey- Appendix Gelding|Satin- Morgan Mare| Teka- Reg. Paint Mare|Shiloh- Reg. QH Mare| Misty- Reg. Pinto Mare