I always recommend starting with a responsive HTML5 site to do your proof of concept. This can help you get some of the server based workflow out of the way. Assuming that the app is designed to run as a cloud based service, you are going to want to have browser access regardless of the platform you build for (iOS/Android/Windows) The advantage of the app is that you can take advantage of pre-built SDK's from the OS which may expedite UX/UI implementation.