Lash of Fire will work very well against plants or zombies, and just slightly better than Sectarus against Skeletons.
Secatarus will work better against Elemental Cloak, Dragonscale Hauberk, Flame Immune/Flame Resistant creatures, the enemy mage in general (cursing an opponent can do far more damage than burns), etc.
The one that is better to cast will depend on the situation.