I know i have chosen a FF for my major pierce weapon (see my Final flourish or Barbarous thron blade thread) but I want a back up elemental pierce.
I would really love a shock flourish but for now I gotta choose from the both
I am the sort who likes to get up close and dirty and run away!
I'd say Rigadoon (as that's my choice) because unlike Magma, you can't so easily spread the fire across multiple targets or the map; you'll attack something, set it on fire...then kill it.
Whereas the stun could actually help make the kill easier, the fires extra dps ticks won't likely change your hit/kill number. Thsi could be especially true attacking such things as Trojans, or Devils who dodge further attacks, allowing the stun to be productive.
A similar theory applies to LD; fire won't help save your life or make the kill easier, but stun certainly can.