I confess to having thought about this myself for many years. The nly conclusion I can come up with is that Batman is, overall, a more intriquing character than Superman. But, historically speaking, the early Superman books are more important due to Superman's "first tier" place in comic book history. After a while, when the playing field has been levelled, so to speak, Superman and Batman are basically in the same weight class. And I think Stan Lee hit it on the head in regards to this. Batman overall is more appealing because he is a troubled HUMAN. Superman, on the other hand, is basically an alien who has had it pretty good.
Superman had a wonderful upbringing. Batman has his personal hell. There is an appeal there that, once the time frame of publication evens things out, makes Batman a character more easily related understood.
But I collect pre-code horror? What the Hades do *I* know?