The final user will see that executing a query, f.e. searching for a song title: postgresql will execute it much faster than mysql. So: Why postgresql isn't supported any more by version 2? postgesql is the best development option by far.
May be you have try with little collection, but with a larger one i know that you'll see the huge difference!

Someone can tell me why mysql and no postgres is a sensible choice?

This article explains it: MySQL in Amarok 2 - The Reality