For a project, I will need to run a certain task in Houdini every minute or so, while other things happen in-between. The task takes ~10-20 sec. I wonder what's a better approach: 1) launching hscript every time (no GUI, but still some overhead), 2) keeping a Houdini session open/minimized, and using Python & RPC server to issue commands.

I would opt for a hython session per task if you can spare a few secs of startup time. Easy to manage and recover from failures, keeping processes running for a long time is imho harder to deal with then firing short new processes. But obviously it's a per case
situation.