what linux OS should I install on a backup notebook if my main one is debian?
I'm your regular end user. I use my computers to edit text, audio and video, watch movies, listen to music, post and bank on the internet...
my main computer uses now debian 12.5 after abandoning xubuntu.
For my backup notebook I have several candidates:
Simply install debian 12.5 again, the easiest choice.
Install linux mint, so I get ubuntu but without them throwing their subscription services down my throat. I'm unsure about other advantages, as ubuntu is debian based, maybe the more frequent program updates? Kernels are also updated more often than with debian as far as I know. Do you know of other advantages?
Go for FreeBSD: this might require a learning curve, because this is an OS I've never used. Are commands that different from debian?
other more niche linux OSs seem too much a hassle and I guess won't be as supported as the main ones.
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.