Both are well balanced moves, but I dont know if I should boost all stats for 1/3 of its health, or boost the already huge attack stat by 6 in exchange for 1/2 of its total health.
You'll become a sweeper if you put on the belly drum which will be pretty good with a salac berry. Then you can get a substitute in for the block and get a salac berry boost. Then you might outspeed the opponent and get a drain Punch and heal yourself up. Afterwards you can have another move like Thunder Punch or Poison Jab. So I suggest Belly Drum if you are using my strategy.
Clangorous soul has less of a drawback and can cover weaknesses as well as strengthen Kommo-o’s strong points.So that would be a better choice. But an alternative is to get belly drum and use a normalaium Z as Z belly drum gives you the stat boost and heals your health. Though of course the choice is entirely yours.