Your main problem with FLAC is that it isn’t supported across all devices. I can’t think of a keyboard that plays them…
I prefer high bitrate MP3 as just about everything plays them. They also can have synced lyrics metadata embedded in the file itself. Something .Wav and FLAC don’t support. Ideally, I can take any music in my library, transfer it to a phone to a tablet, to a computer, or to a keyboard and I don’t have to convert it.
Yes, nowadays, storage is so cheap, to be perfectly honest, why use any compression at all, even if it’s lossless..? But if you want to not have to be constantly changing formats for different devices, good ol’ mp3 is still the gold standard…
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An arranger is just a tool. What matters is what you build with it..!