The aluminum Swift is way more stiff than a Dahon. It's as stiff as any road bike I've used, in fact a bit too much for my tastes; the ride is rather harsh IMO.

One item of note about the Bike Friday that Foldabe Two linked to earlier... it uses 451's, and the Swift uses 406.

I may be wrong, but I think the 451's are "track legal" and 406 is not. I recall seeing how someone used a custom fork on a Swift to get 451's or some other track legal wheels on it, but that was a long time ago....