you should pick the lastest patch (8.0.<highest patch number>). .net 8 installs side, and does not update in place like the 4.* version of net did.
when you run a .net 8 executable, it looks for the highest patch version installed. you should regularly update .net 8 and install the latest.
the sdk install will include the runtime install.
to check installed version:
>dotnet --version
to see all installed versions
>dotnet --info