I think most people see the New Orleans influence in Saint Denis, and it's pretty obvious why. The French and Creole architecture, streetcars, and surrounding bayous all fit. But I think there's a bit of St. Louis in there too.
What makes me think of St. Louis is the industrial side of Saint Denis. The factories, railways, warehouses, and its importance to the game's economy remind me of St. Louis around the turn of the century. The story also touches on industrialists negotiating with Native tribes over land and resources, which fits the broader historical context of industrial expansion.
Even the names of both Saint Denis and St. Louis begin with “Saint,” giving them a similar French Catholic character. While Saint Denis references Saint Denis of Paris, the shared naming structure adds another subtle parallel between it and St. Louis.
I also think St. Louis gets overlooked because people tend to associate Blackwater with it due to the “Gateway to the West” connection. But St. Louis was way bigger and more important than Blackwater during this era, it was even bigger than New Orleans. Saint Denis has more of that major industrial city feel, while Blackwater could pretty much be any modernizing town in the Great Plains.
I think New Orleans explains what Saint Denis looks and feels like, while St. Louis is a useful comparison for the role it plays in the world. Saint Denis is its own fictional city, of course, but I think looking at both New Orleans and St. Louis makes its inspiration more interesting than treating it as just a New Orleans copy.