I considered Potion-Steward at first, but agree that SM-Steward is better. The way I see it, if they collide, Potion-Steward is massive tempo loss, while SM-Steward is perfectly fine. If they don't collide, the idea scenario is that you get a University, but your deck is small enough with Spice Merchant that you aren't very far behind (plus the extra cycling means you can cycle a bit faster as well. The ~30%(?) chance of almost certain death isn't worth the upside.
Steward-Steward just seems worse than either - on collision it's ~Potion/Steward, on not collision you can trash 1 extra card but lose the potential chance to buy a Potion. Could be wrong though.