How To : 3 speed brompton to 2/6 speed one

A question often asked by new want to be Bromptoneers : what speed brompton should I buy? 2, 3 or 6 speed? The answer should be depending on everyone's personal preference. But for a 3rd party upgrade parts supplier's point of view. We will say go for 2 speed. Why? This will make your future modification project has a solid foundation to go any direction.