It entirely depends on your army as a whole rather than anything else. A big part of the game now, which too few people pay attention to, is the fact that unit synergies are vital and extremely powerful.
In my opinion, if you don't have a Slann, the Engine is worthless. If you do, it's okay due to the extra dice you get for using it, but it ultimately depends on how much ranged firepower you have already. If you have little, I'd go with the regular Stegadon over the Engine. If you have enough already, or play a defensive army, the engine is better.
But if you have, say, Kroak or Tetto'Eko, the Engine becomes much better thanks to the rerolls you get that can be used towards it. And Kroak counts as a Slann for the extra engine dice too. But if you have both any Slann and Tetto'Eko, the Engine becomes a must-have. Combining rerolls, the extra dice, and Tet's Curse of Fates on the Engine, the precise control you get over the engine is insane and makes the time stopping roll actually reasonably viable, though still not a surefire thing.
If you do use the Engine though, and as counter-productive as this may sound, don't charge with it. Keep it in your back line with your Slann because the Engine is a support unit despite being a charge-monster, and is a support unit first and foremost.