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I know people who had problems with the 2 speed system.
I don't know people who had problems with the S.A. 3 speed system.
I have the latter, bike is from 1992 and I suspect the rear wheel to be nearly as old as that. Got it second hand in 2007, never had to worry about it.It's heavier though, I'd go for a 2 speed if I was going for a new Brompton.
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2 speed ftw, derailieur works flawlessly (had mine for about 6 years). Never any issues. Chain tensioner is better too imo. Rear wheel is marginally quicker to remove. Plus I save about a kg or so in weight (over the 3/6 speed).
If you do go 3 speed get the narrow range (if you live in a flat area, like london), or wide if you have hills.
Considering a Brompton. What does everyone think of the various hub options in terms of durability?