From what I can tell, the Madison is Rome's top-of-the-line women's binding, while the Milan is Union's second best. Honestly, either one is more than she really "needs." I could get her some cheap Ride VXN or something and she wouldn't know the difference. Anything would be an upgrade from the crappy Roxys she has now, but I figure I might as well get something she can use for the next few years.
From my standpoint, I really don't like the forward lean adjuster on the Milan, which only has 2 settings instead of the usual adjustability. They're lighter than the Madisons though. The Madison also has a full length baseplate cover, while the Milan doesn't.
16 NS West . 15 NS Snowtrooper BIA . 13 Salomon Pow Snake
15 Now Drive . 14 Burton Genesis
14 Flow Hylite