Not sure if that lower saves benefit more here.

Say you've a 15% chance to save against spell, then the spell has a 85% chance to go through. With resistance, it has an 85% of 85% chance of going through, leading to a chance of 72%. This is a reduction of 13%, and also means that on average it takes ~1.4 casts to land a spell, instead of ~1.2.

If you've a 60% chance to save against a spell, you go from 40% chance of being hit to a 16% chance of being hit. This is a reduction of 24%, but more pronounced, the average amount of times you need to cast a spell to land it goes from 2.5 to 6.25.