Torterra and Empoleon both have a high attack stat and powerful STAB moves, which makes them useful in most boss battles. If you really want to figure out which one is slightly better than the other, there are a few differences you can look at.
Torterra's STAB moves are super effective against 7 different types, but both of them are resisted by 3 types. Its physical attack makes it better at using swords dance, which is one of the best status moves in the game.
Empoleon's STAB moves are super effective against only 4 types, and 1 type resists both. Its steel type gives it many resistances that are nice to have in certain boss battles.
Gardenia: Even though Prinplup has the type disadvantage, it learns pluck, while Grotle is stuck with bite and curse. This matchup is pretty even for both.
Maylene: Prinplup has bad stats, while Empoleon is weak to fighting. Neither have a powerful STAB move at this point in the game. Torterra learns earthquake, which is super effective against Lucario.
Candice: Even though Empoleon doesn't resist that many of her moves, all her Pokemon are weak to flash cannon. Torterra's ice weakness isn't very helpful here.
Team Galactic: Empoleon resists the flying, poison, and dark moves that the commanders and Cyrus often use, while Torterra has a lot of trouble dealing with their flying Pokemon.
Volkner: Empoleon is weak to electric. Torterra is immune and its STAB earthquake is super effective.
Aaron: Most of his Pokemon resist both of Torterra's STAB moves, and they all know bug moves. There's only one move on his team that Empoleon doesn't resist.
Lucian: Even though Empoleon resists psychic, 4 of his Pokemon know a move that's super effective against Empoleon.
Conclusion: If you want more help with Maylene, Volkner, or Lucian, pick Turtwig. If you want more help with Candice, Team Galactic, or Aaron, pick Piplup. If you're not sure, either one will be totally fine.