I have the Audi crossbars and they work well for a rocketbox and ski racks. I have not put on a bike carrier but plan on it this Summer. If you do get the crossbars, be sure to install the little rubber grip in the slot, makes a HUGE sound difference. I use my S3 for EVERYTHING and love this little rocket. "Use it" I say.
I also put 4 small patches of 3M plastic film, kind of like invisible bra, under the crossbar pads as they scratch the paint. I only had some minor scratches in the clear before I got around to protecting the paint but after a full winter with them on and off the top, I can see scratches in the plastic so I highly recommend it. If you want the rack on the back I am not sure anyone has a trailer hitch that will fit the S3.
Amazon search for: Scotchgard Paint Protection Film, 84904, 4 in x 84 in