I'd probably just take Engine of the Gods. The regular Stegadon and Ancient have been nerfed a fair bit by 8th edition, and there will most likely be an increase in warmachines, meaning they get skewered earlier than before.
Engine of the Gods is still vulnerable to the same stuff, but it's abilities mean you don't need to be as aggressive (ie there is no rush to charge into combat). Park it behind your Slann+TG block and bomb enemy assault units that attack them (Burning Conflagration doesn't need LOS). Its defensive aura ability is also pretty useful, as with the right positioning you can cover the TG and Saurus blocks at the same time.
That said, if you can keep it alive, a regular Stegadon has a Giant Bow, which is pretty much our only artillery piece, and is useful for ripping wounds off big monsters or killing heavy infantry. The Ancient Steg spams out a lot of poisioned darts, but you already get that from cheaper Skink Skirmishers, so it's kinda redundant.