7 years riding folding bikes here. All boils downt o the following:
Dahon (20" bikes) will ride better and feel more "performance oriented" than Brompton, but will not fold neatly AND becomes a nightmare to travel if you plan on assembling/disassembling the bike several times on the same trip.
Brompton will ride OK, but you will not keep up with roadies, and will not feel aggressive while riding, but will effortlessly fold into a nice little classy package, allowing unlimited adventures without using a tool.
The best? Well, if you need a folding bike to have inside your car and plan on riding a lot, Dahon.
If you plan on riding 10-15 miles EVERYWHERE and want your bike with you as your best friend, Brompton.
Good luck on your final decision.