Given
- the volatility of the platforms Companion and Synch run on (Android and IOS), and
- that they interoperate with Calibre, a moderately complex application that runs on different platforms (Windows, MacOS and Linux), and
- that Calibre is enhanced and updated every fortnight, and
- that Companion was abandoned several years ago and no-one has shown interest in rescuing it, and
- that Synch is maintained and enhanced by an active and reportedly user-friendly developer, and
- I doubt anyone has swapped Synch for Companion since the latter was abandoned, but quite a few have gone the other way,
In my opinion it's a no-brainer - Calibre-Synch.
BR