We're considering the same--three kids, but replacing a 750il
We just test drove the 650 GC and it drives and looks great. Having a 2010 750il for the past 32k miles, I can say it is not my favorite car. It has always felt like driving a turbocharged tank, and the normal comfort setting makes it spongy on corners. Sports mode throws the car into a funny shift pattern, which once put me off track on a corner as it shifted down right at the apex--not the right place for that maneuver. So the 650 GC seems to be a different animal, being already stiffer in the ride and seems lighter as well. They are going to loan us one for the day, but it's a difficult decision as the interior is smaller by about 4 inches in width.
Are the kids going to be fine in the rear? I think so for trips under 50 miles, beyond that, I'd take the Prius v.
Still, the decision is not done as MB is coming out with a new S class that's being early released in March I hear.
ps we also test drove a Panamera Turbo but I can't get over the similarity of its look to the old 944s.