Stable (Debian) means that when you get it working it is less likely to break when you update. A broken installation on a server is quite stressful. Downside for desktop/laptop is that it may not support the latest games and hardware.

Ubuntu is probably more stable than Mint, but less stable than Debian. Which you choose may be more personal preference than objective value.

Yes, but if you are happy with Ubuntu don’t worry about it.

• Open-source purity
• Stable
• Traditional
• Upstream sources for much of Ubuntu

Mint loaded Steam via the package manager and it worked out of the box for me. There have been some games I had to try different versions of Proton with, but I have never found that to be not true for some games.