Answering this question from a lore perspective(Still learning lore gradually btw) :
The answer here isn’t so cut and dry as it really depends on a
variety of factors including talent, power, accessibility, etc. But a way to think about is like this. For a mage, their skill in the arcane is earned and learned over time and can be considered quite powerful in its own right. Not to mention, a mage’s skillset can be more versatile than a warlock’s i.e. fire and ice magic, portals, teleportation, etc.
Warlocks are a bit of a different breed, however; both in the power they wield and how they use it. As a rule, warlocks use the fel, and the fel is an extremely destructive source of power, probably more so than a mage’s spells. And it can be more accessible than a mage’s mana reserves, as the only thing it requires to use is life, whether given or taken. From felfire to summoning demons, in terms of of pure destructive potential and access, I would most definitely say warlocks have the advantage.
However, that power is a double-edged sword. The fel, while powerful, is also volatile and addictive, extremely so. It alters, it corrupts, and it destroys not only its target, but its own practitioners. The fel is like poison, corroding anything in its path that it touches, and a warlock’s power is based solely on such power. The more they use it, the more it alters and destroys what’s around them, including themselves.
So, in terms of whether mages or warlocks are more powerful, it really depends on the practitioner and their skill. Characters like Jaina Proudmoore or Gul’dahn are both powerful users of arcane and fel magic respectively, and because of that they could no doubt beat the average mage or warlock. But answering from a pure viewpoint on their fonts of power, I would say the fel is most certainly more powerful, but volatile and too dangerous to use. Whereas with arcane magic, it is more reliable, powerful in its own right, more versatile, and say it with me, SAFER.
PS: There’s actually a video on how powerful warlocks and mages can be that I found from a Youtuber. Check it out if you guys want, as it helped me gain some perspective on the two classes.