I can only speak to my personal experience with Repetier Server vs Host, which is now several years old. I’ve been using OctoPrint for quite a while now, so have not returned to Repetier recently.
I was intending to use my 3D printer interactively from the computer on which I installed Repetier. The unified installer on Windows installed both Host and Server. Server immediately, and in the background, polled all available connections, and hooked into my 3D printer which was connected by USB. Server took over the port and made it impossible to use Host to connect to the printer (since it was already connected). At the time, my experience and expectations seemed to mirror may folks I interacted with online.