Mezzo is quicker and stiffer than a Brompton in my experience of owning both bikes. It is also easier to get a comfortable ridding position. GEnerally the mezzo is easier to upgrade than a Brompton.
However one issue that a Mezzo is difficult to upgraded cheaply is the under gearing. This can be altered to dual drive with difficulty but great on my black bike. Easier option would be a dual drive crank or a good internal hub gear see the "dual drive on a mezzo" threads. Matt52 claimed to have fitted a shimano cespro hub which occuring to mezzo does not fit, due to chain clearance.
I have ridden a birdie but never owned one. Main advantage a mezzo would be it is easier to manage on a train and are available new for half the price of a Birdie. The birdie is even more upgradable. The Birdie, (and to a less extent a Brompton), is more conformtable than a Mezzo due to suspension and wider tyre options.
Last edited by bhkyte; 06-22-12 at 03:38 PM.