Charging iPhone users more was actually one of the suggestions Phil Schiller made. I'm pretty sure Spotify charges more for people who subscribe from within the iPhone app. They made a lot of noise a year or two ago about this and ended up with this approach.
Personally, I'm very skeptical of subscription options since many companies are bad actors; making it difficult to cancel or throwing adds and promotions at you.
For developers I think a possible strategy would be to sell two products; standard edition and iOS optimized. Charge +30% for the iOS version since Apple users are (supposedly) willing to pay more.