I had a 2006 D7, I now have a 2007 9 (IX) FS. D7 smaller fold, steel frame. 9FS alloy frame, suspension makes for a very comfy urban ride. 9 FS seems slightly free-er rolling, but is heavier. Wouldn't want to carry either very far, but the D7 would be easier. They are both fine bikes. I much prefer the Downtube for its suspension and ride quality.

The quality of the hubs and rims is much better on the Downtube. I had issues with the rim joints not being flush on the Dahon, which I thought spoiled the ride - they thunked when braking. Also the wheel hub cones and BB were way too loose on the Dahon when delivered, easily greased and tightened up though.

No rack or kickstand on my 9 FS.. - I miss the kickstand. I fitted a seatpost rack.

The NS models are lighter.

Dahon will have fixed any issues with rims bynow.