Well, as Tayaya alluded to, it's more that mostly popular anime/pop songs are what's shared; do note that the arcade versions (to my knowledge) don't really "lose" songs over time so they're kinda picking whatever they feel is best for each new version. Which makes the overlap here make even more sense than usual, because they were released less than a year apart, so what's popular has remained popular. And the shared tracks include NGE and One Piece's themes, not to mention Zenzenzense from Your Name which is probably gonna be around for a while now. Similarly I think Sugar Song and Bitter Step's much more popular than you'd think for it being Blood Blockade Battlefront's theme; when I was in Japan a couple years ago it was the hot new thing being added to every rhythm game, so no surprise it's still in both. And the other shared pop songs are popular as well, of course.
As a result of the timing you also have the awkward case of the PS4 having a Kamen Rider theme as DLC (since Build would have either just started airing or been about to start airing at its release, I believe?) while the Switch has it by default.
...I should say all of this is what has me conflicted about which to pick when I do get one, since there's a big overlap in what I actually care about here, but I think I'd still go Switch for handheld for the time being. If anything I just wish the Vocaloid and to a lesser extent Touhou sections were much more expanded. I'd easily lose some Namco Orignals for that; it's probably blasphemous to say, but I can play other rhythm games for original music, hah.
