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The impression I got is that Caleb was unique in being able to absorb and contain The First's power. It's likely that if DW had done that she wouldn't have been able to cope with that power and died.
Either that or been taken over and overwhelmed by the First itself, becoming nothing more than a puppet without any humanity left.
On the whole I would say that Caleb- at full strength- was more powerful, more vicious and more invulnerable. The only way to really hurt and eventually kill him seemed to be being sliced and diced by a magical weapon.
Given the magic texts Darkwillow assimilated in the Magic Box, however, if she'd gone up against Caleb, she may have known that. I agree with the assessment that she might then have become an asset to the First Evil herself, though.