In my experience in socal, DD is a senior and my son is 12yo, it's quite the opposite. I think the level of play for girls softball was higher for 8-12yo, but now that puberty is kicking in, I'm starting to see some early staches and 4in growth spurts. DD has always been on hard charging teams.I've heard cases where a girl played baseball until high school where she switched to softball and ended up playing in college.
The skills are really transferable so you can probably keep her in baseball until she tells you when/if she wants to switch.
At the 8u level it's likely that baseball has better development as well depending on where you live. The level of play for softball can greatly lag that of baseball in certain areas.
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I think a lot of it is that girls possess a bigger attention span at younger ages than boys.
Either way, baseball or softball will be fine for their development.