Explosion does more damage than Self-Destruct, as you can see by their base power difference (250 for Explosion, 200 for Selfdestruct.) However, in Gen V, you do not want to use these moves. Before Gen V, these two moves had the added benefit of halving the opponents defense when used, allowing it to OHKO the foe in almost every case. However, this effect was removed in Gen V, essentially halving their power. No longer will these moves OHKO a majority of foes, and will cost you your Pokemon. Pokemon can learn better moves than these in Gen V. However, in Gen IV and other earlier generations, Explosion is very useful (and Selfdestruct on things that can learn it but not Explosion like Snorlax and Mewtwo.) Even Pokemon with higher Special Attack can use it due to the high base power of the move.