Magnets: the Dahon system is good, but not foolproof. The Brompton fold locks up super-solid when completed. I had a Dahon Speed P8 in bus luggage in its soft bag once--the driver, handling it, ended up having it open on him, severely pinching his finger in the hinge. He was right pissed at me. Not really Dahon's fault here--if he'd held it by the bag's handles he'd have been fine. But it does illustrate the difference.
I have a 2nd-floor apartment and have lugged my Cross Check up & down the stairs for days at a time. I don't love it but it's doable. The Brompton handles is much easier to handle, but I still have to be careful not to bang it into doors and such, especially when I'm also carrying the front bag I use with it.