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 I'll explain how I decide which one to buy. Go through these steps in order, and once you get an answer, you stop.

1. If Tarsier's buildtime can be taken advantage of to deny absorb: buy Tarsier.
2. If Immolite's finesse exploits the enemy or forces them to make sacrifices to avoid being exploited: buy Immolite.
3. If Immolite allows you to spend tech that you'd waste if you bought Tarsier: buy Immolite.
4. If Immolite doesn't force your opponent to buy prompt defense: buy Tarsier.
5. 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.
6. 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.
7. If Immolite forces out inefficient defense (Forcefield or worse): buy Immolite.