If they used to be an awesome badass then the servant is probably weaker, because the container shoves them into an archetype and limits their access to all of their cool stuff at once.
If they were a relative nobody who got famous because they did/created a famous thing, then the servant is probably much stronger, because it gives them baseline superhuman abilities, and that thing they did is now a Noble Phantasm, when it wasn't before.
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There's a lot of other stuff. Most people with Magic Resistance did not have it in life, unless they weren't entirely human.