What are some alternatives?
When comparing Songs2See and Simply Piano, you can also consider the following products
Flowkey - The easiest way to learn piano with your iPhone or iPad
Pianu - Learn to play the piano in your browser.
Yousician - An interactive guitar-learning app.
Playground Sessions - Online piano learning game.
Synthesia - A fun way to learn how to play the piano.
Touch Pianist - Play piano like a pro without playing piano.