Having experienced a speed d7 for awhile, I'd go with the downtube a second time around.
Why?
1) It has an IGH. less fuss and muss with chain lube, and worrying about a derailer getting beating around every time you put it in your trunk. I wouldn't buy a derailer based folder again for my usage. If you never have to lay it on its side, it might not matter for you.
2) Dahon uses alot of non-standard parts. As I've wanted to upgrade various bit this becomes an impediment. It also means that parts for repairs can become unavailable a few years down the line. Downtube uses more standard parts.