The biggest difference is frame stiffness. I've tried dahons, terns and bromptons so there is quite a noticeable difference once you stand up and start wrenching on the handlebars to make the crest of that hill! The frame is the part that you cannot upgrade afterwards no matter how much money you put into it, so think of it as a good canvas you start with and the parts are the paint. You can change paints as you go along but you will always be working on that same canvas.

edit: one more thing for daily riders that fold 2+ times a day, the tern hinges have repleacable bushings and swings pretty much with very little friction, over time when the bushings wear out you can just buy new bushings from tern for a few bucks. But for the dahon once the hinge bolt is loose and has play, there's not much you can do but purchase a new bike, it's a steel bolt pressed into aluminum directly. This only affects commuters that fold them ALOT. My wifes Dahon still folds pretty tight and snug, but it's not a daily commuter.