One problem with the comparisons here is that the Mini model year matters. For some time, the Mini stem folded on the outside of the fold with an aheadset-type of stem. I tried it that way for a few months. For multimode commuting or tight storage I agree that it makes for an unwieldy fold. I swapped the stem for one that sticks the stem on the inside of the fold and decided to live with the shorter reach. With regards to the fold it is much better; although not as tidy/tight/robust as the Brompton/Merc. In my experience, it is pretty easy to keep the Mini's folded package together when rolling it with the seatpost extended: you simply tilt it a bit to one side and the big 16" wheel rolls over any sidewalk/street imperfection.

If it helps, the 2009 Mini sticks the stem on the inside of the fold. It also appears that the stem is further forward such that the reach is greater than the 2007 model.

The Brompton's fold is pretty good. Put on a good set of roller blade wheels and it rolls quite well when folded; although I give a slight preference to the Mini in this regard with the bigger wheel. I would not roll either for say for multiple city blocks. But would easily pick the Mini if I had to over the Brompton. I think that both generally do well when moving around the local metro (subway) station. Overall -- I have written why elsewhere -- the Brompton is also better at carrying stuff, IMO.

I would go with the Mini if you prefer fatter tires, 8-speed hub, and more ergonomic flexibility -- I believe that the 2009 stem is height adjustable. Go with the Brompton if the smaller/tidier fold and better carrying system is paramount. I would stick with the three speed hub and save my pennies for a Schlumpf in the future.