Linux

Unless you're specifically wanting to play with a different OS then Debian again. Makes much more sense to be using the same version of Linux and all the software ypu use rather than potentially different versions.

Also it will be simpler to maintain as everything is the same.

If you do want to play / test another distro then Mint has a low learning curve. FreeBSD is more different but you could easily try it and switch to something else if you don't like it. Its different but not so much that linux users would feel totally lost.

Probably the most confusing thing for linux user trying FreeBSD is that Bash is not installed, and BSD uses sh instead by default. Bash can be easily installed and set as the default shell which will give a lot more familiarity. But otherwise it'll feel like a familiar modern complete system, and you can use the same desktop environments you're familiar with already in linux.

EDIT: You did say "backup" in your title. If that's the main use case then definitely Debian again. If your laptop breaks or is stolen it makes sense to have a familiar system to pick up. Also important to sync and backup your data so it can be picked up on the other laptop. If backup machine is your focus then I'd say same OS and look more into data retention and retrieval between the two laptops, and ensure your important data is continuously backed up.