Immolite is an interesting unit. In a vacuum, it's very slightly stronger than Tarsier, but in practice, it can be much worse or much better depending on the circumstances. Here's how I decide which one to buy. Go through these steps in order, and once you get an answer, you stop.
If Tarsier's buildtime can be taken advantage of to deny absorb: buy Tarsier.
If Immolite's finesse exploits the enemy or forces them to make sacrifices to avoid being exploited: buy Immolite.
If Immolite allows you to spend tech that you'd waste if you bought Tarsier: buy Immolite.
If Immolite doesn't force your opponent to buy prompt defense: buy Tarsier.
If Immolite forces out efficient defense (Aegis, Polywall, Plexo Cell, etc): probably buy Tarsier, but feel free to buy Immolite if it's more convenient.
If Immolite forces out medium-efficiency defense (Wall, Innervi Field, etc): probably buy Immolite, but feel free to buy Tarsier if it's more convenient.
If Immolite forces out inefficient defense (Forcefield or worse): buy Immolite.