Flat: Easiest and fastest to hookup and unhook, but the selection of cars that can be towed four down isn't big. Some modifications to the car are required, and one should also invest in a brake system. There's not much to stow away while parked.
Dolly: Loading and strapping takes 15 minutes once you do it a couple times, but does require some kneeling. (Get one of those foam knee pads.) Pretty much any FWD car can be dolly towed without modification. Get a dolly with brakes. While parked, one has to put the dolly somewhere, but they're small and can be wheeled by hand, so this really isn't difficult.
I tow a Prius on a dolly. It works very well and loading/unloading is pretty easy.