Race or gender?
I think that electing a woman President would have the more important impact, because of the international implications. Our civilization's main global adversary right now is (I'm going to be blunt here and omit the usual cowardly and dishonest hedges) Islam -- the most misogynistic and most crudely male-dominated culture on the face of the planet. Electing a woman as our national leader would send a clear and even subversive message to Muslim women (and men!) about what kind of society we are, sharpening one of the profoundest contrasts between our culture and theirs.
If our main enemy were a sort of USSR-sized version of apartheid-era South Africa, then electing a black President of the US would have similar significance, but that isn't the case.