I'm trying to figure out which Pokemon is more useful for my team, Boltund or Manetric?
They are both just electric types with high speed. Manetric can learn ice fang which deals with ground types and Boltund learns high-power physical moves like play rough and wild charge.
Which one is capable of beating more gyms, rivals or any challenge in the game?
Their stats and movesets are similar but either way I'd run Volt Switch as their speed stats are great but defenses are poor. Sending them out first and Volt Switching out if the match up is unfavourable. Manetric if you want it to hit a bit harder or Boltund if you want added speed but just using them in-game, Manetric should still be able to out speed most opponents.
My preference would be boltund due to a higher base stat, higher speed, and balanced attack stats with special attack at 90 and attack at 90. Boltund also has an ability that increases the power of biting moves and whenever one of his stats increase his special attack increases.