Okay, so a 33% reduction, rather than 50%. That's definitely better, in my eyes.
I don't suppose there's any chance that Magnus' bonus is actually a 20% reduction, not 80%?
My gut says that Magnus will be for building units, not upgrading, and that the 50% reduction card for upgrading will be even more valuable in saving resources. Although if resources are counted when you first start building a unit, I could definitely see a strategy of queueing a couple units with Magnus in a city with discounted resources, and then swap in Victor before they actually finish.