The first decision you must make when considering which Brompton to buy is which type of handlebar is right for you. This will largely depend on the type of riding you wish to do as well as the riding position to which you are most accustomed.
Most people choose the mid height M option for a comfortable and fairly upright ride. Others who are more accustomed to riding mountain bikes or road bikes prefer the sportier S version. If you intend to ride long distances, perhaps touring, then P type may be the choice for you.
M TYPE - Classic handlebar shape, offering a comfortable ride.
S TYPE - A lower, more sporty ride position.
H TYPE - Same classic style as the M Type but with a higher stem, perfect for riders who prefer a more upright riding position.
P TYPE - A versatile handlebar offering the flexibility of several grip positions.
Once you have made your handlebar choice you can move on to the choice of gearing.