FYI folks,I sent an e-mail to Brompton customer support and got a reply:
"Thank you for getting in touch. Yes, the current 6 speed set up would be slightly lighter then the 5 speed."
So there it is from Brompton;the 3spd hub/2spd der is lighter than the 5spd hub.
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C'dale BBU('05 and '09)/Super Six/Hooligan8and 3,Kona Dew Deluxe,Novara Buzz/Safari,Surly Big Dummy,Marin Pt Reyes,Giant Defy 1,Schwinn DBX SuperSport,Brompton S6L/S2E-X/M6L-X/S12 T Line