Personally, I think both moves are on somewhat equal footing. Hammer Arm does do a huge chunk of damage, even to some bulky steel types that can shrug off most attacks, but it lowers the speed and people would expect hammer arm over endeavor. I mean if you wanted to run a trick room set than hammer arm is definetely a good option, but if not then your Pokemon is kind of screwed once the speed gets too low.
On the flip side, endeavor can do a ton more damage to the opponent and is a lot less predictable. Certain Pokemon can easily get to low health and use endeavor, quick attack, and then just get another hit on the next Pokemon to come out. Sadly, the only real way to use the move is to have the Pokemon built around the move (E.g. low health, a fast finishing move, study ability, etc) and in double and triple battles the Pokemon could get killed before it can even use the move. In addition, any multi hitting moves like bullet seed or barrage will get stopped by a focus sash or study, but will make the Pokemon faint by the next hit.
Id reccomend Hammer arm, especially in double battles where other Pokemon can cover for the lack of speed, or the speed could help with a trick room. Endeavor is good too, but it can be easy to avoid, and once the opponent knows you have the move, your Pokemon is at a huge risk. Hope this helps!!!