Im still very new but the Slann Starmaster seems like the better option, yet I see several lists with Lord Kroak instead. I do not plan on using Engine of the Gods.
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.
For me, if you are using Trog and not EotG then id go Kroak. if you are using EotG than the Slann is almost necessary to make it reliable. however... all this could be changing in like two weeks... I am currently holding off on building anything more until the release. granted im newer and have like 1500-1800 points total, so it makes more sense for me to pause than for someone that already has 3000+ already built.
With the new edition and changes to the balewind a starmasyer can cast 4 spells like kroak while bringing the slann battle traits, and a constellation.