When comparing Yousician and Piano Maestro, two things jumped out at me.
Firstly, one has an interface that’s clearly designed for children, and secondly, neither is the best way to learn the piano outside of real in-person coaching.
Now, that doesn’t mean that these apps aren’t useful, because they are. However, I want to briefly mention an alternative I feel I must recommend when discussing anything to do with learning the piano. You can find that at the bottom of this page.
First, let’s see some footage on Piano Maestro:
Here's a marketing video of Yousician:
Piano Maestro is clearly designed for kids.
That being said, anyone can use it and benefit from it, if you can get past the cartoon feel. You can download the app here, and try it out for yourself.
In terms of upgrading to some of the in app purchases, here is a quick run down:
$11.99 per month, or $59.99 per year.
The fact with all these "free apps" is that if you want to get all the songs, all the levels, and use them without restrictions, you will need to upgrade. The free versions let you try a sample, but be prepared to make a purchase if you want to get anything out of these tools.
Yousician is a more advanced app.
The interface is very easy to use, there are a ton of lessons and progressions to make, and the software can actually track your performance and give you feedback.
You can also download the app for free, however as I mentioned above, be prepared to buy the full program. You can read more about whether the premium version is worth it or not here, however I'll give a quick summary of the prices:
$9.99 per month, or $119.99 per year.
Which Should You Use?
Because of the way these apps are marketed, you can actually download the free versions, and try them out, so you have nothing to lose.
That being said, I did say that I would recommend a piano learning tool which is better than these apps in my opinion.
That tool and program is called Playground Sessions. You can find more information about it here.