Among folders, the Brompton seems to be hard to beat on durability and steel doesn't seem to be the magic bullet either as even Bike Friday is currently blaming issues on the "conflict in regular bike design between light weight and durability". On the other hand, the Brompton has more nonstandard parts than any other folder I can think of and simply cannot compare to the Mu SL's lightweight wheelset in acceleration.
Perhaps mulleady would like to comment as I believe he's been sporting both bikes for some time now, but I guess the lesson is that you can't have your cake and eat it too.