Update:
Thank you, everyone, for your comments and the discussion.
Now that I got my hands on Crusader, 33ilvl artifact and the Truncheon, I was finally able to carry out some tests.
The artifact is slightly superior in DPS, but the difference is very small, around the margin of 10 (175-180 with Truncheon vs. 185-190 with artifact). Did multiple tests both with a timer and set amount of total damage, for example 5 minutes, 10 minutes; 50k total damage, 100k total damage.
The main thing that I've noticed is that Truncheon provided me more Crusader procs. The short term bursts were insane, especially when Crusader procced along with the haste from the mace. Also, keep in mind that you gain 3 bonus socket slots from the shield + mace combo over the artifact, which is potentially 18 extra stamina and 12 extra armor.
Since the goal of my paladin is to stack stamina and stay alive as long as possible while silencing, stunning and tanking enemies, I believe that I'm going for Truncheon, mainly for the 3 extra socket slots and unexpected damage outputs when haste proc stacks with Crusader. If you're planning on taking on a supporting/damaging role, artifact is most likely going to be better (if you're stacking str+haste or str+crit over stamina, for example).
To conclude, I believe that the artifact is more stable in terms of DPS and short fights vs. squishy enemies. However, since protection paladin (the way I play the class) heavily relies on Crusader for damage and survivability, Truncheon was superior.