So a question I've often struggled with, although it's not terribly important in the grand scheme of things, is which tertiary weapon to give my Tyrannofex. Obviously the Rupture Cannon is the only realistic choice for the main gun, and Cluster Spines is a given for the secondary, but the three flamer weapons are all similar enough that it makes choosing between them difficult. However, after mathhammering things out a bit and thinking things over, I've come up with the following:
(Incidentally, if, for some reason, you take a Tyrant with the thorax swarm, these could also apply there as well.)
Dessicator Larvae (2+ poison) are the overall best choice. They get the same overall numbers as SSB against MEQs and are better against Orks. Their ability to consistently harm all sorts of targets is very valuable.
Electroshock Grubs (S5/AP5) are very good for killing Guardsmen, Guardians, and Fire Warriors, but actually perform worse than DL against Orks and other T4 targets with a 6+ save. They are a reasonable choice, especially if you really, really hate fighting IG, and have a small advantage in being able to occasionally hurt a tank as well.
Shreddershard Beetles (Rending) are only superior against Terminators and Sisters of Battle. Like ESG, they also have a small bonus in being able to randomly hurt a vehicle sometimes, and are marginally more effective in this regard. They end up being the worst of the three, though.
So, in summary, you will pretty much always want Dessicators, unless you expect your Tyrannofex to be hanging next to IG, Eldar, or DE troops very often, in which case I really envy you.
