I have owned and ridden both. For commuting I find the Brompton hard to beat. I often fold it up to carry it on the train, and use it to explore towns outsise the city. The limited gearing is fine for my current riding style. The Brompton is responsive, the three-speed version I have shifts promptly, it is fun to ride.

The Birdy offers a roomier cockpit, and a remarkably smooth ride. It also uses standard components, so upgrades and modifications are easy. I upgraded my Birdy with a 10 speed XT driveline, this made it capable of tackling the steepest hills, while also allowing me to pedal down the hills as well.

The Birdy is a faster bike, it is lighter, and has more gearing. However, the Birdy is a little prone to squeaks, rattles, and creaks. These are usually easy to track down and fix, but the Brompton doesn't seem to be bothered by them. If you live in the city, and your commute is fairly level, the Brompton is a great choice. If you like longer rides over more various terrain, the Birdy would be more suitable.