Bear in mind you're not limited to the gearing options Brompton offers. There's an SA-8 which is almost directly compatible (just needs the forks widening a touch) and also an 11-speed. I'm not a fan of the clunky twin-lever system of the 6-speed so opted for the 8-speed twist-grip, which offers a slightly wider range, smaller steps and a far slicker shift.
Bar-wise, try them all and see what suits, but as others have said, the S-bar does significantly limit your luggage options.