the skiller will add at least 20% more damage to lightning because lm by itself adds 12% each point, and each point in lightning adds 10%. so, the only way the 5/5 would even be close is if the -resist adds 17% more "real" damage, which isn't likely.
for the change you are suggesting, you are only gaining 12 fhr. i doubt you'd hit a meaningful breakpoint with that.