I don't think it's *just* weight of the rider, but probably also the average power that goes towards the "twisting" of the back of the frame. I'd imagine Paulo Bettini, who is short but one of the best sprinters there was (*swoon*), would need something wickedly stiff and overbuilt despite his light bodyweight. Guys, on average, can put out more power and could use the stiffer/heavier layup, but then they can be jealous that we get the lighter frames.

Just saw this today, too, which is not directly related, but brings up some of the proportional geometry that Specialized aims for:
http://www.thebirdwheel.com/specialized-womens-bikes

I'd say try both bikes, if possible, and get out of the saddle and sprint like hell in a big bad gear. If you find the back too "wobbly", then you may need the Roubaix and assume that you may need shims/stem/saddle adjustments. Or if you don't plan on sprinting much in your rides and you find the women's geometry more comfortable (I sure do), go Ruby. Frames nonwithstanding, I find that it's even the details like the narrow handlebars and closer brake levers that make me happy.

-- gnat! (on my second Ruby, big fan)