Neither is better. It really just depends.
I like using Ice Breaker for most PvE activities. The regenerating ammo means I never have to waste time reloading, and it's the hardest-hitting weapon capable of being used at safe distances. I like to get things done quickly and efficiently, so Ice Breaker is my go-to exotic for general activities like Daily story missions.
I mainly use Black Hammer against enemies with lots of health and large critical hit points in an environment where I'm relatively safe and won't have to move much. Examples would be against Sardok in the Shrine of Oryx mission, Aksor in the Winter's Run strike, the Goliath in the Cerberus Vae III strike, or Atheon in the Vault of Glass. Additionally, Black Hammer is also my default special weapon if I want to use an exotic weapon other than Ice Breaker, such as Gjallarhorn.
I have never used Black Hammer, but I have used Ice Breaker, and it has been pretty effective in the Crucible.
I remember getting a Triple Kill by getting a headshot off of one enemy, which caused him to explode, dealing damage to two of his nearby teammates and killing them. (Their health was low, and they were clustered together, which made it pretty easy)
It was a pleasant sight.
Anyways, it has been pretty effective for PvE. The ammo regeneration perk is very helpful.