While I don't think FF: FS makes as many glaring mistakes as Supes '25, Supes '25 is just a more interesting film, IMO.
Corenswet's Superman stands out more than any of the FF, and Sue was the only one who could put up any fight in that regard. Supes '25 had the superior supporting cast, and I'm not sure how even the most ardent First Steps fan could disagree.
The world of Supes '25 feels more fleshed out, and Gunn uses that world to make timely social commentary. While FF's alternate 1960s setting is cool, it ultimately feels like window dressing, and nothing more.
I liked both movies, and neither are stone cold masterpieces. That being said, I can see why Superman '25 has had stronger BO legs.