For burst damage lightning does more damage and hits instantly (actually before the weapon damage)
So for Double strike and para blow use lightning.
For disruption since spell interrupt chance isn't based on damage done staggered damage works out better than burst damage, so Magic Arrow or Fireball work really well. However since damage is delayed, if you proc your spell after landing a para hit, the delayed damage will break the paralysis.
Also in double proc events, (Double Arrow, or Double strike) even if you proc both times you won't and get 2 effects, the damage will only be dealt once. Magic arrow delay, sound effect, and animation also much shorter than fireball so tend to have less rubber band effect.
Have to check, but I believe slayer damage is interrupted by spell procs, though that may just be on instant spell procs like lighting and harm.