I would suggest that you give each a try and see what you like.
I use a light slip cover, which is so thin it can fit into a pocket or purse, and keeps the grease off. But a Brompton folds in such a way that it protects you from the chain. With a Dahon, you have to keep to one side of the bike. You can get one for either bike.
In the past, Dahon's quality control has been weak, and it's unclear whether that's been resolved. However, 14R and others note few problems with the Curve so far.