Today, Harper has the better matchup with an OPS of .889 against Irvin (9 plate appearances). That's compared to Alonso's OPS of .730 versus Rea (9 PA). That's a pretty big difference, highly consider going with Harper.
Weather impact
Heavy wind (blowing out), hot temps, ballpark elevation, etc can all have a big impact on long fly balls. An extra few feet can be the difference between warning track and a 3-run dinger. In Harper's game, our HRForce Index at first pitch is 1.1 (10% more HRs expected). Meanwhile, Alonso will be playing under an HRForce of 1.2 (20% more HRs expected (roof likely open)). So, long flies will travel a bit further in Alonso's game. Weather edge goes to him.
Weekly Roster Lock Leagues
For those of you in leagues that require you to set your lineup before the start of each week, day offs can be a bummer. Everything else being equal, give the edge to players that play at least one more game a week. For the M-F week that includes 9/29/24, Harper has 6 games scheduled, while Alonso has 5. So, with one more game, that could make a difference, especially in H2H leagues where you need to pile up points. Slight advantage to Harper.