In short.
Bromptons are brilliant folders.
Dahons Ride well.
Birdy has suspension.
People will tell you how far they've ridden on a Brompton, or how well their Dahon or Birdy folds or how smooth their Dahon or Brompton is on poor surfaces.
It's up to you but remember you are buying a dual purpose item. All dual purpose items are a compromise. The trick is to pick the balance between functions which suits you bast.
I own a Dahon Matrix-V 26" wheel folder that is not at all commuter train friendly, a Dahon Speed-Pro which rides almost as well as my full size tourer and folds small enough to take on a bus and a Phillips Boardwalk for taking on holiday which folds slightly smaller than the Sped-Pro and is cheap enough to park anywhere!