We have pair of three speed Bromptons, we store them in the car when travelling. Because they as so compact we can get them in the foot well behind the front seats. It's not a problem storing the 3 speed on its side as all the gears are enclosed in the sturmey archer can in the hub, I suppose the only thing to watch on the 6 speed is the derailer system is exposed.
We manage ok with the the 3 speed as the gearing is so well spaced, however I am sure the 6 speed could make life easier, the nice thing with the 3 speed it is very basic and easy to use. We have the Ikea bags and they fit a treat..
They are a great little bike and very well built, my wife was very unsteady on a conventional bike but she loves the Brompton which has given her confidence, they are very expensive however they hold their value very well so you should get your money back if you ever sell them.
I am sure there will be a 6 speed Brompton owner to advise..