I agree with Susie, in theory a front-to-back bi--polar trode should work best for a woman, rather than a side-to-side one. Especially paired with the Stroke routine on an ET-312, which sends a signal that is interpreted by the brain as moving from one trode to the other (front to back or vice versa). However, everyone has different experiences. You should experiment to find what you like best -- and that will probably change over time, and even day to day.
One of the most interesting experiences I have had was with a side-to-side bi-polar vaginal trode in the 6-12 o'clock position that was left running on low power for about 2 hours (I don't remember the rhythmic mode) while we watched a movie. Afterward, her G-spot was as swollen and hard as half of a walnut shell, and simply touching it led to an instant orgasm. Seeing this, I immediately switched to intercourse -- which was a totally amazing experience for both of us. non-stop instant orgasms during intercourse. How many women experience this regularly?
I'm always stunned to realize that virtually no American women have ever even tried estim.
Here's one site for a variety of trodes, including the E-Stim Systems stuff that comes from the UK:
http://www.happystim-usa.com/catalog/c6_p1.html
Others to try: Kenstwistedmind, ExtremeRestraints, MEO (Germany) E-Stim Systems (UK), and many others. Don't skip standard medical electro-gel-pads. They work extremely well on both men and women. You can't put them over hair, and it stings too much if freshly shaven or waxed. Smooth skin allows placement of gel pads anywhere, but you need to wait a day or two for micro-breaks to heal. Medical websites sell pads for a fraction of the price on sex websites.