The curve weights about 2-3 kg less. If you plan a carrying up stairs, around train stations ect you will notice the difference.
The Envoy uses a derailer system, vs the internally geared dahon. IGs are much easier to maintain and command a much higher price. See numerous discussions of the subject.
Also note that curve is 16" and Envoy is 20". See numerous discussions for the implications.
see sheldonbrown.com for general cycling discussion.
If you do not like it, I would think the dahon will have a higher resale value.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.