Disclaimer: I have a Boardwalk D7 - predecessor to the Speed 7, and I have never seen or ridden a Downtube bike.
That said, DT is Aluminum frame and Dahon is 4130 Chromoly steel - the DT Bike is lighter - not getting into the Alum vs Steel argument.
The DT Max rider height is 6'2" - Dahon 6'4" - I am 6'3" (barefoot) and have a 36" inseam. I have the seats on both my Dahons at MAX height. There's also the stretch-out factor - my Boardwalk has the current tilted forward design and gives enough reach - the DT NS looks like it has less distance from seat to handlebars. Don't know your physical size.
The DT appears to be geared lower than the Dahon. 28"-81.5" for DT vs. 34"-94.5" for the Dahon. So, terrain will matter a bit. DT is 9-speed & the Speed 7 is just that, 7-speeds.
Price: Dahon is MSRP is higher than DT, but they are sold for less many places, you just have to look.
I'm sure others can point out additional differences.
The owner of Downtube just posted to this very thread.
On the other hand.
It will take probably three emails from you to get a brief "half" answer to any question you might have from the Dahon folks.