User blog comment:IamJakuhoRaikoben/Chapter 456: Order, Review./@comment-25085824-20151012144241/@comment-7418318-20151012175959

Considering how Erza is among the strongest in Fairy Tail even though most (but not all) of her magic is Holder Magic and how Mystogan, who technically isn't even a mage, was on S-Class level, I'd say it's a matter of the individual and maybe the item they use, rather than a broad generalisation of Holder vs. Caster.