For slashing attacks (most common with skills such as Power Strike and Slash Blast and Dragon Fury), Pole Arms are slightly better. Spears deal more damage with stabbing attacks, such as with skills like Spear/Pole Arm Crusher. It has been debated whether Spear/Pole Arm Crusher or Dragon Fury is the better mobbing skill for during 3rd job and the consensus (as I understand it) seems to be that Dragon Fury is the better dpm option. Because of this it is recommended to use Pole Arms during 1st, 2nd & 3rd job. Once you reach level 120 and become a Dark Knight, it seems like most DrKs switch back to the Spear since they are going to use Spear Crusher for bossing and grinding with Berzerk. Tl;dr: Pole Arm during 1st, 2nd & 3rd job. Spear during 4th job.
If your initial question concerned only Dark Knights, I misunderstood it. I'm no expert on Dark Knights unfortunately. Wish I could direct you to someone who is. Chances are you have a better understanding of the strengths and weaknesses of the class than I do.
For me is depending on training area... whether or not psb or spear, as long as i 2hit i will use that. So for example if i am at ulu 2, i use psb. but if i am at ob4 (my core training area), i use spear. for bossing, only spear u may want to go through this thread for all the information u need: https://royals.ms/forum/threads/dark-knight-guide.28010/
