Tigr insects is an AOE slow. On spell description, it slows more than single target Cloud of Gummus ... and it costs less mana?
Cloud of Grummus hits Disease resist, and so will land on targets that are completely resistant to regular Magic-based Turgur's/Tigir's Insects.
No, don't "make the switch". Use correct tools for correct jobs.: Nearly all the time, it is correct to use Turgur's Insects. It casts fastest, it slows the most, it lasts the longest. When "being AE" is more important than being strong, then Tigir's. When "the good spells don't work", then you use the one that does work (Grummus).