Invoke Fear vs Trepidation

[Grombar]

For easy reference.
http://wiki.project1999.com/Invoke_Fear
http://wiki.project1999.com/Trepidation

Ok, so Im having a hard time seeing the justification for Trepidation over Invoke fear.
Invoke Fear is 80 mana less and lasts 1 tic longer all while at a neg 20 MR check vs a neg 25 for Trepidation.

Am I missing something? is that extra 5 MR check THAT important? I just don't see myself ever using Trepidation over Invoke Fear.