Android apps that contain ads are clearly labelled in the Google Play Store. The text "Contains Ads" appears right at the top of each listing by the title. There is no such standard in Apple's App Store, and this could easily be remedied by Apple if they cared.
Like others here have already stated, I'm happy to pay for apps. One-off purchase, subscriptions, whatever. I used exclusively Android since the first device (T-Mobile G1) and finally switched to iOS earlier this year, and this has been the biggest obstacle so far. I'd like to install more apps but my phone is pretty much stock since I have no way of knowing whether apps are full of ads.
I am more than a little sceptical of Tim Cook's fiery "data industrial complex" speech last week as he's still not yet ready to take that first baby step and begin warning privacy-conscious users about adware. How do you overlook this as CEO? The other security features on iOS are first-rate, why deliberately overlook adware? I don't get it.