939 chips are in demand due to people who are wanting to upgrade yet not ditch their DDR memory for DDR2, and/or don't want to lose their AGP video cards. Socket AM2 requires DDR2, which is a significant cost difference if you're upgrading, and DDR2 offers ABSOLUTELY no performance advantage, nor does Socket AM2.
Because of this, anyone upgrading a single-core A64 system that already has good memory would needlessly spend easily $100 to $400 extra (depending on how much memory they have) to go to AM2 rather than 939 and get no benefit. Better to spend just a few bucks extra, and get the 939 dual core rather than AM2.
AM2 doesn't really future-proof you either, since there will be no truly revolutionary chips on AM2. AM2+ will support some future chips, but will be limited. The next socket type will come out before the revolutionary chips do - so AM2 isn't even an advantage for FUTURE upgradability.
Simple - if you're buying all new, AM2 is okay. If you're upgrading, 939 all the way.
Oh, and the opty is the better buy.