You could also rent a brompton from NYCeWheels in Upper East Side. In this way you would know very well that the Brompton is a right investment or not? Although I already know the answer...
I think 3 speed is the right choice! And Brompton's resell value is high. You won't regret the amount of money you spend on this folder as the time goes longer and longer. This is how I feel. Just go to rent one for couple weeks and you will know how to make this decision.