Thank you all for the advice.
Because there is not a Brompton dealer anywhere nearby, I don't have the option of trying out all the various possibilities. The one test ride I did was part of a vacation to the Pacific NW that won't be repeated soon. I have decided that I will order the extended seatpost. Here's my reasoning:
1) The telescopic post may have some nice features, but I could eventually upgrade to it if the cheaper option doesn't meet my needs.
2) The extended post will certainly be long enough for me.
3) Someone on the Brompton yahoo group suggested that there is only about an inch difference between a standard seatpost in a 'high' position as an extended one adjusted to be 'low'. Comfort is more important for my planned usage than an inch.
4) The extended post can be chopped to the perfect length, or replaced with the standard as necessary.
*Whew*, that's settled, now I just have to decide what color I want...