Now here's a challenging one. On one hand, we have a 6/6 trampler that can not only do a decent impression of
Frontline Medic, but can also serve as an anti-wrath even if it isn't in play. On the other hand, we have a dude who can't be blocked and MOST of the time, will only need to actually hit twice before you win. They're both pretty powerful and demand near-immediate answers. If you are looking for the more powerful one, I think it depends on what you want to support. Michael Bay's Elesh Norn encourages a more aggressive plan since it's protecting all the dudes you played before it. Throw in some discard or graveyard shenanigans and it becomes very apparent that this thing will be amazing in any deck turning more than one dude sideways each turn. Scuttling Doom Engine comes off as more of a stand alone guy. He offers a lot less interesting interactions and while both dudes could be amazing control finishers as well as aggro curve toppers, the Doom Engine doesn't feel like it is encouraging a player to play aggro as much as MB'S EN. But if you run a cube that has a heroic/enchantment theme, then Doom Engine DOES encourage other players to "build a better creature" so they can strip the Doom Engine of its ability to avoid blockers.
Overall, I guess it comes down to what you want to support. If you're cube has a theme of using the graveyard at all and you want to push players to play aggro, I'd say go with the Phyrexian beast by Bay. If you have a cube where enchantments and auras matter, then maybe the Doom Engine. If you have a sacrifice theme, either is good at dying but killing your own bomb shouldn't come up that often. In my personal opinion I like the Soul better most of the time. It all comes down to what you want at the end of the day, though.