Redirect to application if installed, otherwise to App Store

I think the more simple answer would be to set up a page on your server with the following javascript:

(function() {
  var app = {
    launchApp: function() {
      window.location.replace("myapp://");
      this.timer = setTimeout(this.openWebApp, 1000);
    },

    openWebApp: function() {
      window.location.replace("http://itunesstorelink/");
    }
  };

  app.launchApp();
})();

This basically attempts to redirect to your app and sets a timeout to redirect to the app store if it fails.

You could even make the code a little more smart, and check the user agent to see if they are an ios user, an android user, or a webuser, and then redirect them appropriately.