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tamhas - I'm agnostic as I use neither
Calibre Companion has a free version, limited to 10, or maybe 20, books - which is enough if you only keep current and a few unread books on your device.
Calibre Cloud also has a free version, not sure what the limitations are - looks like you need Pro to use Mantano as the reader.
Whilst they fulfil a similar need, their approach is
very different. So I suggest you try both of them and decide which best fits your needs.
Here's a recent thread on the feasibility of incorporating Calibre Cloud-like features into Calibre Companion ==>>
Support Calibre database on cloud? . It also has a link to an older and longer discussion.
If I was to use either I would probably use Companion because I don't like to be cloud dependent as I'm often offline. But I only keep a few books on my devices hence I don't really need it, besides I prefer not to connect my devices (android) to calibre.
BR