Wow! what a difference!
Flowkey is superior in every way. You play whole tunes and not some simplified version of them. You get to see the hand movements on top of the screen while you play which names the note at the same time. This helps you to learn the keys.
The songs are original covers and not some measly version of them.
You can choose left and right hand singularly to play when learning songs and then both. At 50%, 75% speed and wait to play note modes.
The musical theory is much better. There are scale practice modes.
Overall, flowkey towers above the simplypiano app.
I have started from scratch all over again and learnt so much more on FK.
Simplypiano did lead me in though, but I wouldnāt recommend paying any money to it. Go for the best. I feel I have wasted 6 months on it.
I am trying to master āimagineā by Lennon and it is taking some time. It is intermediate though, but at least it is the real tune this time.