Wellllllllllllll, the Brompton folds smaller. A bit. If you're on a packed commuter train, a bit makes a difference. Both fit in the back of a city car.

The tiny Brompton fold is of interest to the nerdy. Include me in that. The Mezzo fold is wider, and the handlebar-post hinge sticks up. The Mezzo though, looks and operates like an alloy-framed 21st century bike which was designed from scratch to be hugely stiff and light. The Brompton is an evolution of a 1979 or so handmade steel bike. The Mezzo's stiff double wall rims and spar alloy frame are a bonus. It's a fast bike.

The weight of the bikes is as similar as makes no difference. You won't want to run far with 11Kg.

The ride quality depends on what you prefer; geometries and materials are very different, and so is the ride.

So ya got old Brit 'gaspipe' hand-bent traditional design and manufacture, vesus CAD and advanced metallurgy, backed up by Formula 1 suspension guru Jon Whyte expertise. The new Mezzo D10's (curvy frame) use hydroformed alloy, about as advanced as bike frame design gets. The Brompton frame got a face-lift a few years ago, but is basically much the same as things were thirty years ago. Some people like that: Schwinn v. LaPierre, perhaps.

One important difference is that Bromptons are cult bikes, and in the UK retain their used prices. They are seen as Very Jolly British, although almost all of the parts are imported. They are assembled in the UK.

The Mezzo, (as this is a Mezzo thread) is How Folding Bikes Should be. The Brompton is How Folding Bikes Used To Be. Take your pick.