Fun comparison for my old app that I am discontinuing.
iOS market share is around 30%, Android is cca 70% (Europe matches world-wide in this case, my app was area-specific in Europe)
Yet I had 82% installations on iOS.
Also, iOS users are more likely to spend money on apps (mine was free, although it allowed access to paid content).
Not sure, if my numbers are representative, but if I had to do it all over again (I hope not), I would choose iOS first from the business and number of users perspective.
I don't know about F-Droid/Obtainium/Aurora numbers, but I guess it would be less than 5% of installs if the app is available on Play Store.
My recent problems have been with Apple, but it does not make sense to develop the Android app either.
The article summarizing my decision is here: