Why would you not use EotG? if your going Troglodon it does free up more points but the EotG is stronger.
Advantages of the Troglodon:
- Trog doesn't take up a hero slot.
- Trog can dispell
- Trog is cheaper
- Trog is more reliable/less random in what it'l actually do
- Trog has a much clearer role thant the EoTG with its mess of effects.
- Trog influences enemies bravery
Advantages of the EoTG:
- Better stats
- Bonus effects can be better, though luck-dependent and situational
- It's a hero so it can take an artifact
Depending on what exactly you want to do the trog can be quite a lot better.
As for the Original question of slann vs Kroak.
Kroak can be used on a balewind to nuke everything into oblivion. A slann has significantly less firepower.
A slann has the advantage of either bringing double teleport or knowing the starseer and starpriest their spells.
The extra firepower from Kroak is more noticeable though, which is why he's picked more often I guess.