Stick with the nailboard unless you find CBM Hydraulic Muscles or mutate Insanely Strong.
I recommend only using a sledgehammer if you have a strong terrain advantage, a bash skill above 6, good armor, and 15-30 strength.
Alternatively, if you’re facing a single zombie trying to come through a window, then you can take a free shot at him and switch back to the nailboard, the sledgehammer has a high minimum damage, it’s just slow to use.
Once you are up there though, its the most devastating melee weapon around, on a critical you can flatten (almost?) any enemy in the game. You can even gib zombies if you have a high enough bash skill.