Vancouver, Washington and Ocean Shores, Washington, USA
2 - 2007 Custom Bike Fridays, 2 - 2009 Bike Friday Pocket 8's, Gravity 29'er SS, 2 - 8-spd Windsor City Bikes, 1973 Raleigh 20 & a 1964 Schwinn Tiger
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.