People underestimate the value of paying a designer for a high fidelity mock up and building a waitlist to see if anyone actually wants your product
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Real answer: iOS if you have the expertise, Web otherwise
iOS : Network effect and people are willing to spend money way more than they are on Android. But development talent is harder to find than Web
Web: Not quite as sticky for the kind of product that gets used on phones, but super cheap and easy to get talent for it.
Android: Worst of both worlds, no one pays for anything, development talent is not as plentiful as Web and I think maybe even more limited than iOS?
Source: Worked on top Android and iOS apps "back in the day", currently an embedded Android engineer working on HMI