I can back up Thor's assertion that the Speed Uno does actually look really really nice in the shop and on the road. Thankfully, my dealer here in Canada does have a few Mu Uno left plus some Speed Unos, so I could compare both against my Dahon Mu SL.
Speed Uno is a much better buy,because by adding say a S2C dual coaster hub laced wheel, it brings the price up a bit but still less than a Mu Uno. While it is a little heavier than the Mu, the slung over weight using my Dahon Carry On sack is not noticeable. It is noticeable only when you lift it up unfolded. Weight is not a concern riding mostly on flats.

My wish is for Dahon to introduce the Dahon Speed Duo at the same price as the Speed Uno, but equip it with a S2C hub and low direct gearing plus slightly higher hub gearing for decent hill climbing and flat riding. I think that will sell better and I don't need to mod and have a spare wheel. Dahon or Tern, are you listening?!? I have money to buy.

I rode both the Mu and Speed Unos briefly and I can say that the Mu is slightly stiffer like my Mu SL. The Speed is good too, but unless you're used to attacking a hill with a 53T/19 on a 700c road bike which I do quite often, you will not notice a difference in stiffness. Besides, it is not a good idea to ride standing up with folder bikes which you will do more often with these single bikes if you are not already a strong rider. If you are already riding mostly with a 53T chain ring on the hill and or cruising up with a 39T with your road bike, then I suggest getting the Speed as you'll be doing more sitting down riding the single speed bike. If you're going up the hill with a 39T and or a 34T on a compact chaing ring on a regular road bike and wanting more granny gear, then get the Mu as you'll be standing up more and would appreciate the added frame stiffness.

Hope this helps.