Depends. 125fcr is generally better - but for some setups its not possible.
My supersafe HC Warlock can tank Tier 5 Doll Heralds in full games. If you want that, you gotta go 75fcr.
75 is worth it for carefree farming with no risk, even in SC imho. The ability to ignore your screen, multitask, or leave the room is underrated.