Playing around with the DPS/TDO spreadsheets, the difference (that I found at least against mewtwo) is about .4 at absolute most, and given that gengar bearly survives past the first charge move if that, SC Gengar's 'optimal' dps isnt achievable given few to none have that perfect break/bulk point legacy gengar. not to mention chandelure does have some bulk, or at least some compared to gengar so we have something like a houndoom tyrantiar situation, theoretically one's better because of moves, but in practice the other really is because it lasts longer.
Also VS confusion/Psychic mewtwo, Chandelure has a full .65 DPS up on SC gengar