Ask how the product will actually be used
A responsive web app works everywhere with no install, updates instantly, and is usually faster and cheaper to build first. A native mobile app is the better choice when your users need offline access, push notifications, camera/BLE/GPS hardware access, or when frequent, habitual use makes a home-screen icon genuinely valuable.
Discovery and distribution differ
A web app is discoverable through search and shareable with a link — useful for content, marketing sites and tools people use occasionally. A mobile app depends on App Store/Play Store discovery or your own marketing to get installed, but once installed, it has a much stronger presence in daily use through notifications and the home screen.
Cost and maintenance
A single web app usually has a lower upfront cost and one codebase to maintain. Native iOS + Android apps mean either two codebases (native) or one shared codebase (Flutter) plus ongoing App Store/Play Store compliance work as OS versions change every year.
A common pattern that works well
Many products launch as a web app first to validate demand with lower cost, then add native iOS/Android apps once there's a clear case for offline use, push notifications, or daily habitual engagement that a website can't deliver as well.