I realise that this is an old thread but I want to add a comment because I know there will be newcomers to folding bicycles looking for advice here.

Brompton and Dahon/Tern are not the only choices. But I argue that the Brompton design is a better investment.

The placement of the main hinge in the design of a folding bicycle is *crucial*. Dahons and Terns folding in the middle of the frame. This causes flex when you're riding and the hinge will degrade if the bicycle is used for commuting. Also consider that the weight limit of the Dahon/Tern design is about 100 kg. You'll cause more flex if your weight is near this limit and you carry a backpack when you ride. At the time of writing, Dahon provides a five-year guarantee. Your opinion may differ, but when I spend money on a bicycle I want it to last more than five years.

The Brompton hinge is like that of the Bike Friday and Ori/Mezzo. The rear triangle swings under the bicycle:

https://www.youtube.com/watch?v=pNnOdoUn3kg

https://www.youtube.com/watch?v=dyiDWxnq5LM

It's a more durable design. Bike Friday has a model of folder that is rated up to 120 kg.

The small wheels of the Brompton make it easier to transport (smaller result after folding) but the 349mm wheel (16 inch) is rare in North America.