XV-21's "new and improved brush" lets hair get wrapped around brush so you need to clean it more often. (You do get the original brush as well but you will have to change it out) The filter on the XV-21 is better, but you can mod the XV-11 by gluing on a hepa filter.
Ideally you need one XV-11 and one Roomba for each cleaning area (Try to break down your home into multiple areas) XV-11 goes first to suck up hair and the Roomba goes next to work the edges.
You also need at least 2 mints for hard surfaces. I don't have a Scooba at the moment, but will add if I can ever find one at a deep discount.