I have ridden several Bromptons and I test rode a Birdy Speed and a Brompton S-type back to back.
The Birdy in my humble opinion was far superior in ride, comfort, speed and build quality.
It is like comparing a Morgan or Lotus 7 to a Porsche.... all desirable machines but very different.
The Birdy is not much better than the Brompton. People tend to make absolute statements like that when both these folder's are very context specific.
The Brompton is a wonderful commute and go bike. It doesn't have suspension like the Birdy but it folds quicker and more compact and is a very fun ride. The parts are high quality and very durable and there is ample proof of that from Brompton customers who use one regularly like myself and also in the high resale value that Bromptons fetch. If you chose a B then get the 6 speed version, it has very nicely spaced and efficient gears. Don't like the shifting levers but the gears themselves are good.
I don't agree the Brompton is a bad ride but I do agree with the opinions of the other users that the Birdy would be a more versatile investment for your needs for on and off-road trails. it also has a compact enough fold for your needs. The Brompton is really only suitable for normal road riding, that's what it's designed for. I recommend the Birdy City with the Shimano Nexus 8 speed hub. See:
http://www.foldingbikes.co.uk/birdy_city.htm