The 2, 3, or 6 speed options will all work well (although I would get the reduced gearing - Brompton low gears are still kind of high). But if you are unsure, you should just get the 6 speed (-12%) - it costs much more to retrofit than it does to buy new because you will need a new wheel (and maybe a new hub).

Also, even if you don't need six speeds for around town (and you may not), it's pretty easy to travel with a B., so you may find yourself in hillier locales.

But, seriously, if you don't know, get the 6.