It is true Na2Cr2O7 is more soluble than K2Cr2O7.
CrO4−2 Is tetrahedral:


Na2Cr2O7 is not the primary standard but K2Cr2O7 (acidic) is the primary standard reagent for titration because sodium dichromate is Hygroscopic in nature, that means it absorb moisture from atmosphere, that will cause incorrect reading in titration.