On the basis of calculations using density functional theory, we investigated the relative stabilities of all isomers of Sc3N@C67B and Sc3N@C67N as well as those of stable isomers of Sc3N@C66BN. As a result, we predict that Sc3N@C-68 can be doped substitutionally with a boron atom much better than C-60. This effect can be ascribed to the favorable electrostatic attraction between the encased Sc3N cluster and the polar C-B bonds of the fullerene cage, which show the important role played by the encapsulated atoms in stabilizing the fullerene. A difference in the interaction also determines the regiospecificity of Sc3N@C67B. On the contrary, N-doping of the fullerenes forming Sc3N@C67N is much less favorable than that in C-60 or C-70. A judicious choice of stable isomers of Sc3N@C66BN among a vast number of possible isomers indicates that Sc3N@C-68 can also be doped with a pair of B and N atoms better than C-60 under the simultaneous existence of B and N sources. Relative stabilities of various isomers of the BN-substituted fullerenes can be understood in terms of the combined electrostatic effects in the B- and N-substitutions of Sc3N@C-68 complemented by a specific local preference in the N-substitution and the formation of a B-N bond.