The difference between 1.25 and 1.50 is 20%. So you will do 20% less damage, leach 20% less hit points.
I use undead slayer with 100% fire damage vs changelings. I'm not sure that undead slayer helps but 100% elemental damage is very important.
I use arachnid slayer (100% physical) vs spiders. They are weak and easy as well as wolves.