Two if (carrying) weight is an issue (you can easily save ~800g), otherwise a 3 speed SA hub will bring you MANY miles of troublefree cycling. I currently have a 3-speed. I do not have a lot of hills here and i am a spinner not a masher, i rarely get to use the top gear, so every once and a while I wonder if a 2 speed might not have suited me better, though it is nice to have the top end for those few moments. I commute on my Brompton daily (~14km), I've taken it with me on various trips on trains, etc. with or without luggage and it had worked out fine.

-Carcinogent

P.S. If you are mechanically inclined you can replace the internals of the 3-speed with old medium ratio ones (Sturmey Archer AM), found on ebay, etc. and get 3 very nicely spaced gears. (Roughly 2 tooth difference (~15.5%) in each gear step) Also with a 3 speed you can change the rear cog if you are not happy with the gear ratios you have; with a 2 speed i think you have to change the crankset.