Newer sets also tend to trade slightly better since people still need cards from them. HGSS still has quite a few tradable cards though. DCE is always valueble, as is pokemon collector. Triumphant as a whole though seems like a much better set, and I would rather pull just about any rare out of TRI instead of the Azumarill/Hitmontop/others from HGSS.
Triumphant seems like the way to go. 3/5 of the trainers are valued highly, most of the primes are good, and due to cities being far off you don't have to worry about certain cards not living up to their hype. Gengar prime will actually appreciate so long as lost zone comes out in the forseeable future.