We don't really have enough information on either combatant in the feats department, but if we look at the opponents each of them were pitted up against, it becomes clear who's intended to be stronger. The Wood Dragon was used to subjugate a full powered Kurama in the past, and now it's fighting Bijū Mode Naruto. Manda II doesn't have a prayer of defeating either of them.
But Vin Throa above me brings up a good point: the Wood Dragon might have the power to suppress Bijū chakra specifically (I don't agree it can suppress any type) which makes its "power" vs. Kurama irrelevant.