I'm going to try to keep this short-ish, since I'm actually hoping to play Game 1 when I'm done. One thing I should note after the last post is the different park factors in the Polo Grounds vs. Robison Field: the Yankees enjoy a 30% boost to home runs and about an 8% boost to batting average vs. Robison Field providing a generally more neutral environment and cutting batting average and home runs by 4%. So, the Yankees huge power and run-scoring advantage could be partly illusory. However, that makes its pitching look even better.
On a player-by-player basis, New York has a much more balanced team than St. Louis. The best player on either team is Rogers Hornsby of the Cardinals, who was NL MVP in Alt-1917 and is an excellent bet to repeat in 1918.
I suspect the list of best players after Hornsby in the series is mostly Yankees, though. Oscar Charleston is my choice for #2, though sorting by 1918 Wins above Replacement (WAR) puts Peckinpaugh* 2nd and Charleston 3rd.
The list of Yankees/Cardinals non-pitchers with WAR over 4.0 in order are Hornsby (STL), Peckinpaugh (NYY), Charleston (NYY), Pipp (NYY), Baker (NYY), Heilmann (NYY), Hyatt (STL), Zimmerman (NYY, injured), Pratt (STL), Santop (NYY), Gilhooley (NYY). So that's...10 players (not counting Zimmerman, who can't play), and 3 of them are on St. Louis.
The pitching is less clear to me, though again the Cardinals maybe hold a slight edge? Hendrix and Hoyt (both on the Cardinals) are far and away the pitching WAR leaders, and/because/but they racked up a lot more innings. Pennock had a similar year for the Yankees in terms of ERA+, FIP, and all that business, but pitched 100 fewer innings due to injury and my aversion to overworking my pitchers (even in 1918). I don't know what the Cardinals AI** will be doing, but I plan to keep managing in a more modern style.
I'm reluctant to make a prediction for all sorts of reasons, but I guess I will. I think the Yankees will win, I'll say they do it in 6 games. But it wouldn't be a massive surprise to me if they swept or lost. How's that for confident? :)
*As noted here in other posts, the way OOTP does WAR seems to give an enormous boost to shortstops who aren't butchers. While I suspect Hornsby finishes #1 in WAR even without it, Peckinpaugh in particular doesn't always pass my eye test even though he's perennially among the league leaders in WAR.
**Jack Hendricks was the real-life manager for the 1918 Cardinals, he's available in OOTP (I'm not playing with coaching staffs on) so we can say he's managing here, too. Miller Huggins would potentially have made more sense--he was Cardinals manager from 1913-1917 after retiring from playing for them in 1916, but in this universe he was traded to the Terrapins/Orioles and retired. So maybe he's their manager?