In Torchlight 2 for a berserker with a shield, what's better: howl or battle rage?
Howl reduces the efficiency of enemy mobs in different ways while battle rage significantly improves your power against enemy mobs. Both skill's efficiency is also directly proportional to the number of mobs around you. Given that you use a shield I would assume your character has high defense and could capitalize with drawing lots of mobs thus both skill is pretty much very usable when engaging a pack.
Do consider when you are up against bosses, some bosses fight alone while other summon minions albeit not many, and at higher tier you would not want to prolong your engagement with a boss. Between battle rage and howl, battle rage has a significant efficiency loss the less number of enemies you engage so if you would be considering ease of battle against bosses over quick runs with mobs you may want to invest more on Howl.