These are just my rough opinions
Rend Heaven
Pros: Higher activation rate
Cons: Damage boost is lower, so sometimes it won't matter. Is largely useless on magic units since not many enemies have magic to begin with. Physical units also don't get a boost against pure magic units, for that. It also doesn't help much against Dragonskin enemies
Dragon Fang
Pros: High damage added. It depends on the units themselves but it's capable of turning a 2hko into a 1hko. You get it at Level 10/0
Con: Lower proc rate. Low availability.
Ignis
Pros: Adds more damage than Luna/Rend Heaven if you have better stats. (Which is to say, not really that much)
Cons: DLC skill, so it cannot be bought or inherited. Most characters do not have any magic stat, and the magic users don't have "a lot" of strength either, so the pro is unlikely to happen. Also has the same con of not being very useful against Dragonskin enemies.