Mercenaries are very useful for holding ground and dealing out loads of damage, but they are a turret and literally all abilities have an activation or Chanel time. Power techs are very mobile can grapple and with right spec boost to them, then there is all the instant abilities and elimental damage to wear the enemy down. Downside nearly all abilities are 10m range in other words close quarters.
Basically it comes down to either range or combat. I went power tech for pvp caus it's nearlly always close quarters and merc for pve. Hope this helps