Cubasis is, in my opinion, the app to use if you are looking to produce music in a professional DAW using your iPad.
Good news! Version 3 is currently deeply discounted and they just released an Android version!
What's New in Cubasis 3.2 | Steinberg-->
Musical ideas come when you least expect them and are often gone in a flash. With Cubasis, you always have your mobile DAW with you, because Cubasis is available for all your mobile devices. iPad? Check. iPhone? Of course! Android smartphone and tablet? Sure thing! And now we bring one of the fastest, most intuitive and complete mobile DAWs to the world of Chromebooks, too. Welcome Google (again)! Welcome Chrome OS! Cubasis is ready to capture your ideas whenever and wherever you are.