I own both. The GSA is more versatile and has earned my loyalty. If I had to give up one it would probably be the K, but it would be a rough decision.
Regarding passengers, for the driver they are less noticeable on the K, but both bikes can get a little top heavy when stopped. If your son weighs over 140 (a guesstimated threshold) the rear will sink and you may be more prone to wheelies. The K feels more like a Harley with a passenger, no sag, but the passenger seat is high up so it gets annoying when they move around a lot at stops.
My wife prefers the GSA for passenger comfort. She actually finds the seat (stock) therapeutic for her back. The K has a gap between the seat and backrest that is uncomfortable for her. Others complain that the top back shakes. The 2012 GTL models have a very tight space for the hand grip. Supposed to be fixed in the later 2013s.
My son (who is 5'9") didn't like the GSA, because he felt the space was too small and made us too "intimate". He wasn't that wild about the K either, but mainly because of the grip rails.
A stock GSA will haul more stuff than the K. On a recent trip I had a fully loaded K ready to go, but had to switch bikes because the windshield on the K was acting flakey. Took everything out of the K and put into the GSA with room to spare.