Some shop owner I spoke to, who sells and rides both bikes (and others) puts it somehow like this:"to me the birdy is the best all-rounder. If I was to own only one bike it would be a birdy. It is capable of all. But the brompton is unbeaten in what it is designed for and I use the brompton a lot more often"

?Do you go off-road: birdy
?Do you go in bus/train/restaurant: brompton
If nothing of the above both bikes will be different and their own kind but equal in quality and capability to do what you want, if fitting to your anatomy.