Knockdown/Blunt beats pierce any day.
Free hits are better.
True that you will get free hits from knockdown, but you don't count the hits you won't get because the warhammer is a lot easier to follow and a bit slower, compared to the steel pick. Question is, will you get more hits in with the invisibility and speed of the pick or the knockdowns with the warhammer. They're both good choices to be honest.
On a side note. Does hoplite use 1h or pole arm proficiency?
Polearm