Played a few games with both now, and the Stegadon feels more impactful.
I am now firmly in the Engine of the Gods camp. I never really played around with them before 3.0 but after a couple of games there's a lot to like about them.
It's 40 points cheaper than a Chief, and while you lose a good ranged attack, some throwaway spear attacks and a great command ability, you gain 2 extra wounds (making you a little harder to bracket), the potential to fountain mortal wounds, and are almost as killy as the Chief at a baseline level anyway.
The major downside is that a Slann is a must-have to get the most out of it, but he or Kroak is likely to be a mainstay in most of my lists anyway. You'll need to play him quite aggressively in order to stay within 12" of the Engine. Seems risky, but I found that my opponent is too concerned with the rampaging Steg to focus on the Slann anyway.
In my experience, the Engine was able to sit in the throng of combat, get activated twice per shooting phase (which was more often than not spitting mortal wounds at everything within 12") and then still deal out good damage in the combat phase later.
Seeing as I was also dropping Comet's Call on him in the hero phase, my opponent was in complete awe at the volume of mortal wounds I was able to hand out.
I think I still need to play around with the Chief some more, but for now I think I'll be basing my first tournament lists for Thunder Lizard around an Engine (maybe even two...).
