There is no rules reason you'd need to pick the same dragon type for both.
Certainly it would be thematic to do so, but from a purely mechanical standpoint, picking two different dragon types gives you more resistances (one all the time, from your race, and one that you can turn on with a sorcery point). And since your breath weapon wouldn't benefit from Elemental Affinity (as it's not a spell), there's no real benefit to overlapping.
