What is the difference between the iOS subscription for Reader @ 69.99/yr and the online Acrobat Pro subscription @179.88?
You can use the first only in Acrobat Reader on iOS or Android.
The first one unlocks the premium features only for mobile devices like iOS/Android on the Acrobat app. But with 2nd subscription, you can use some paid features like edit PDF, export PDF, etc, on the desktop app as well as the mobile app.
Here is an FAQ document on the Acrobat premium mobile subscription for more clarity.
Let us know if you need any more information.