I don't know if you saw this on the Xtracycle site, but under the women's Townie model they have a link that says "a word on the ladies frame." It says:

The ladies’ frame’s very low step-over height is convenient for stopping and for getting on and off. That’s especially helpful in town, with stops at stores and when you have your S.U.B. loaded. Women and men can equally appreciate the convenience of avoiding the Giant Leg Swing.

BUT… While perfectly capable of handling regular loads, the geometry of a ladies’ frame is not as strong as the diamond frame. If you think you’ll be carrying 100-pound-plus cargoes with frequency over the life of your bike, you should get the diamond frame. This goes for all S.U.B. models and brands we sell.

So you should keep that in mind as you decide between the men's and women's models.