My Birdy was stolen several months ago, and I decided to replace it with a Brompton. The Brompton is quite a nice bike, and very good for short commutes and sidewalk riding. The ride is smooth enough, and it folds quickly and easily.
in comparison, the Birdy was a bit lighter, (I had the older straight-frame version), has a larger, more comfortable cockpit, and much more easily upgradeable than the Brompton. I was easily able to fit XTR V-brakes, levers, and a 10 speed shifter, derailleur, and cog. The best thing about the Birdy is that I was able to set up the seat, bar, etc. to the same dimensions as my 56cm road bike.
The Brompton is a great grocery-getter, and is good enough for my regular work commute, but the Birdy was quick enough, and large enough to be a potential replacement for my fullsize roadbike. If you are planning longer, faster rides, the Birdy is a great choice. The front and rear suspension tames rough pavements quite nicely. For all around fun, the Brompton is a great bike.
i live in Japan, and the Birdy is something of a cult bike here. You can probably buy one through Rakuten, which is a Japanese internet retailer which sells pretty much anything. Rakuten now provides English support, and shipping from Japan is generally cheaper and faster than from most other places.
Good luck, and I hope you enjoy whichever bike you choose.